Releasing satellite images from January 15, the Institute for Science and International Security said in a report Wednesday that the pictures showed the early construction of a fourth military nuclear reactor set to be the same size as two of the other buildings.

"Pakistan is determined to produce considerably more plutonium for nuclear weapons," ISIS said in its report, noting that since the announcement of its first reactor at the Khushab site in 1998, the nuclear power began constructing a second reactor around 2000-2002, and began building a third in 2006.

Pakistan has reportedly doubled its nuclear arsenal over the past several years, increasing its stocks to more than 100 deployed weapons.

Only four years ago the country's arsenal was estimated at 30 to 60 weapons, but has since stepped-up its production of plutonium and highly enriched uranium.

"They have been expanding pretty rapidly," ISIS president David Albright noted late last month, the Washington Post reported, with Islamabad edging ahead of its nuclear-armed rival India, estimated to have somewhere between 60 to 100 weapons.

"We want major nations to start negotiations on reducing the number of such weapons. It's no secret that such weapons are deployed near us, in Kaliningrad. And to our east as well," Jukneviciene told Lithuanian public radio.

Rumours have repeatedly surfaced of the presence of such arms in Kaliningrad, a Russian territory sandwiched between the Baltic Sea, Poland and Lithuania.

In November the Russian military dismissed US media reports that it had moved short-range -- or tactical -- warheads to Kaliningrad earlier in 2010 despite pledges made as early as 1991.

"It's in our interest that so many arms -- including tactical nuclear weapons which present a threat to our existence -- are not amassed all round our borders," said Jukneviciene.

Lithuania and its fellow Baltic states Latvia and Estonia watch hawkishly for Russian military moves. Moscow only withdrew its troops from their territory in 1994, three years after they won independence when the Soviet Union crumbled.

The Baltic trio, with a total population of 6.7 million, have rocky relations with giant Russia, notably since they became anchored in the West by joining NATO and the European Union in 2004.

Lithuania hailed Russia's recent ratification of the updated START arms-reduction treaty with the United States that covers long-range missiles.

Washington is keen to launch talks over short-range weapons that have remained uncovered by previous nuclear disarmament agreements with Russia.

But on Monday Moscow said it was premature to set a date for a new round of talks.

It argued they could only begin once Washington was ready to reconsider its position on a new missile defence shield for Europe, which foresees anti-missile facilities in former Soviet satellite states now in NATO.

The subterfuge work was probably started in the 1990s without raising suspicions, according to diplomats who have seen the report which could be discussed by a special Security Council sanctions committee in coming days.

The report is based on evidence from US scientist Siegfried Hecker who was taken to a secret site by the North Korean authorities last November.

Hecker has told of seeing hundreds of centrifuges at the Yongbyon complex when he went there. The report quoted him as saying that North Korea must have had foreign help to build the facility, one diplomat, speaking on condition of anonymity, told AFP.

"The report says that the centrifuges at Yongbyon must have been operated at at least one other facility before being moved there," the diplomat added.

"The equipment must have been started at least in the 1990s and had to have been made with outside help," said another diplomat, also speaking on condition of anonymity because the report has not been made public.

The panel of experts were set up to monitor two rounds of sanctions ordered by the UN Security Council against North Korea because of its nuclear bomb program.

"They said that more North Korea entities and individuals should be brought into the sanctions regime," said the diplomat.

The new names who would face a travel ban and assets freeze could be added to the sanctions regime without the need for a new UN Security Council resolution that China would probably oppose.

The United States and other western nations on the 15-member Security Council have already called for tighter sanctions.

The panel met Hecker after his visit to North Korea. None of the experts have been allowed to Yongbyon or any other North Korean facility.

North Korea has already said it has a plutonium bomb. It says it carried out bomb tests in 2006 and 2009.

It admitted that low-enriched uranium was being made at Yongbyon with the centrifuges after Hecker's visit. The North Korean authorities insist the uranium is for a "peaceful" light water reactor.

The UN experts made 10 recommendations, including adding more North Korean nuclear officials and companies to the sanctions list. They said North Korea state entities are using aliases to get equipment and that a wider selection of materials should be put on a list of banned imports, the diplomats said.

Neighboring countries should also tighten export controls to the North, the report was quoted as saying.

North Korea kicked UN nuclear inspectors out of Yongbyon in 2002 before withdrawing from the Non-Proliferation Treaty, the pact banning the spread of nuclear weapons.

The report did not name any country suspected of helping North Korea but the West has accused Iran and North Korea of working together on missile technology.

Rebecca K.C. Hersman, deputy assistant secretary of defense for countering weapons of mass destruction, told American Forces Press Service the effort will allow the agencies to leverage their expertise and resources for “maximum effect to President Barack Obama’s nuclear security agenda.”

In April 2009, from Hradcany Square in Prague in the Czech Republic, Obama called for reducing the number of nuclear weapons in the world and building a new framework for civil nuclear cooperation. A year later at the Nuclear Security Summit here, the United States and China agreed to strengthen cooperation in nuclear nonproliferation, nuclear security and the fight against nuclear terrorism.

Also at the summit, Chinese President Hu Jintao committed to building the Center of Excellence on Nuclear Security outside Beijing.

According to an Energy Department fact sheet, the agreement paves the way for its National Nuclear Security Administration and the Defense Department to work with Atomic Energy Authority representatives in China to create a central site for training in all aspects of nuclear security.

“In many ways, DOD is the supporting player here to the broader DOE objectives,” Hersman said, “but DOD brings strengths to table, particularly in … site security, transportation security, incident response [and] inventory management, as well as experience in developing and providing training and curricula for nuclear security.”

DOD and DOE have worked together in the past, she noted. “These are all things we have done on multiple occasions directly and in support of DOE,” Hersman said, “so we see this as a natural fit for the [Center of Excellence] effort, which is expected to incorporate all these elements.”

The center will serve as a forum for exchanging technical information, sharing best practices, developing training courses and promoting technical collaborations to enhance nuclear security in China and throughout Asia.

The two-story center will be financed through a U.S.-China cost-sharing arrangement and is expected to be complete by 2012, said Dave Huizenga, principal deputy assistant administrator for the office of defense nuclear nonproliferation in the Energy Department’s National Nuclear Security Administration.

“This cooperation is largely with the nonweapons side -- the Chinese Atomic Energy Authority, which runs their civilian research facilities and has a role in their nuclear power facilities,” Huizenga said. “But the hope is that if we share best practices and this information gets to [the nonweapons] part of the Chinese nuclear sector, the defense people will benefit from it indirectly.”

The National Nuclear Security Administration has had an ongoing partnership with the Chinese since 2005, Huizenga said.

“We’ve had a robust best-practices sharing exchange of information on physical protection and guard forces and materials control and accounting -- all the things you do to make sure that nuclear materials stay in the facility where they’re supposed to be and aren’t moved off illicitly,” he said.

The agreement has taken such cooperation to a new level, he added.

“We’ve had a small facility where we’ve been doing this training since 2005,” Huizenga said. “But we want to consolidate everything into one larger mock-up training center so we can bring Chinese and others in the region into a state-of-the-art facility where they can get hands-on experience understanding what a guard would do if an alarm went off on the fence on the perimeter of a nuclear materials storage site, for instance.”

The center, Hersman added, also likely will offer the following:

• Training nuclear site personnel to measure and account for nuclear material and to design and install nuclear material security systems;
• Training protective force personnel using scenario-driven threat-response exercises;
• Training personnel on international nuclear safeguards requirements and inspection techniques; and
• Environmental testing of nuclear security system components.

According to a White House fact sheet, the U.S. and Chinese governments have cooperated since April 2004 to enhance nuclear security under the Peaceful Uses of Nuclear Technology Agreement.

In 2005, the U.S. and China sponsored a joint technology demonstration at the China Institute of Atomic Energy outside Beijing that featured established nuclear security and international safeguards technologies and illustrated nuclear security best practices.

Since 2005, experts from the United States and China have conducted more than 15 workshops on nuclear security issues and activities.

“China has always described the center as supporting regional and International Atomic Energy Agency nuclear security cooperation,” Hersman said, “and we strongly support that goal.”

Citing US non-government analysts, the newspaper said that only four years ago, the Pakistani nuclear arsenal was estimated at 30 to 60 weapons.

"They have been expanding pretty rapidly," the report quoted David Albright, president of the Institute for Science and International Security, as saying.

Based on recently accelerated production of plutonium and highly enriched uranium, Islamabad may now have an arsenal of up to 110 weapons, Albright said.

As a result, Pakistan has now edged ahead of India, its nuclear-armed rival, The Post noted. India is estimated to have 60 to 100 weapons.

Top US disarmament negotiator Rose Gottemoeller suggested that it might be the best opportunity for more reluctant states to keep leverage in drawing up a Fissile Material Cut Off Treaty, which is supported by the major nuclear powers.

Pakistan this week reiterated its lone public opposition to starting negotiations on a treaty in the Conference on Disarmament, after two years of deadlock in the consensus-bound 65 nation body.

"Let me just place full emphasis and priority today on my main message, which is to launch the negotiations this year on a Fissile Material Cut off Treaty (FMCT) in the Conference on Disarmament," Gottemoeller told journalists.

"I think that is a kind of general timeframe," she said, while adding that it was not a "specific deadline".

The US Assistant Secretary of State for Arms Control told the conference on Thursday that waiting indefinitely was "not a viable option."

"If we cannot find a way to begin these negotiations in the Conference on Disarmament, then we will need to consider options," Gottemoeller said.

She nonetheless insisted that Washington preferred a deal through the Geneva-based body, stressing that its consensus rule gave all countries equal weight -- including in defining the scope of fissile material rules.

"I for one hope that Pakistan will take this as a serious effort to verify their concerns," the US official added in a media briefing.

Washington also supported starting discussions by technical experts on the sidelines to lay some groundwork on an FMCT in the meantime.

UN Secretary General Ban Ki-moon warned on Tuesday that the deadlock in Geneva threatens international security by increasing the risk that weapons might fall into the wrong hands.

The FMCT is widely regarded as a key building block in stemming the spread of nuclear weapons.

Pakistan, an atomic weapons state, fears neighbour India's bigger nuclear arsenal. Islamabad also wants to deal with existing stockpiles of nuclear material.

In the opening session of the 2011 Conference on Disarmament, Pakistan's ambassador Zamir Akram sharply criticised reported moves to bring its neighbor into the Nuclear Suppliers Group (NSG) and other bodies that allow trade in nuclear materials, including for weapons.

"Apart from undermining the validity and sanctity of the international non-proliferation regime, these measures shall further destabilize security in South Asia," Akram said.

"As a consequence Pakistan will be forced to take measures to ensure the credibility of its deterrence. The cumulative impact would be to destabilize the security environment in South Asia and beyond," he told the 65 nation conference.

He said Pakistan maintains its opposition to negotiations on a ban on the production of new nuclear bomb-making material, a lone public stance that has blocked the Conference on Disarmament despite pressure from major powers.

US disarmament ambassador Laura Kennedy told journalists last week that negotiations on a ban, a Fissile Material Cut off Treaty (FMCT), were a priority for Washington.

"We believe that this is long overdue, it's a priority. And this sense of urgency is not, again, simply one of the United States, but is widely shared," Kennedy said.

Akram earlier told journalists that Pakistan "would like a treaty that deals with stocks not just future production."

Nuclear powers broke more than a decade of deadlock in May 2009 by agreeing on a work plan at the world's only multilateral arms control forum, which can only make decisions unanimously.

The plan included full negotiations on a fissile material ban, as well as talks on nuclear disarmament, the arms race in space and security assurances for non-nuclear states.

However, the disarmament conference has slumped back into deadlock since then, as Pakistan raised fresh objections.

"We believe that we need to build a capacity that is a credible deterrence at the lowest levels," Akram explained earlier, adding that Pakistan would nonetheless not seek to entirely match India's nuclear capability.

UN Secretary General Ban Ki-moon was expected to push for progress in the conference during a visit to Geneva on Wednesday.

Diplomats widely regard the FMCT as a key building block in breaking the spread of nuclear weapons.

But non nuclear states as well as some countries with smaller atomic arsenals insist on parallel progress on the other issues, especially full nuclear disarmament by the big powers.

Parts of the Geneva plan, including the fissile ban, underpinned an agreement between 189 nations including the major powers at the UN's nuclear non proliferation (NPT) conference last May.

The two-day talks in Istanbul between Iran and the six world powers ended without progress and no new meeting was scheduled to tackle concerns that Tehran is secretly developing an atomic bomb.

"We had hoped to embark on a discussion of practical ways forward, and have made every effort to make that happen. I am disappointed to say that this has not been possible," EU foreign policy chief Catherine Ashton told reporters.

"No new talks have been planned," she said.

Ashton headed the delegation of the P5+1 group, comprised of Britain, China, France, Russia, the United States and Germany.

In London, British Foreign Secretary William Hague said Iran's "refusal to engage" was "extremely disappointing".

French Foreign Minister Michele Alliot-Marie said it had been the conditions Iran had set -- including the lifting of sanctions -- that had scuppered the talks.

"This blocked everything," she told reporters in Paris.

A senior US diplomat said diplomatic channels nonetheless remain open.

"The door is open... We continue to believe that there is time and space for diplomacy," he said on condition of anonymity.

The United States and Israel have not ruled out military action against Iran to prevent it from acquiring nuclear weapons.

Iran had set the stage for fierce wrangling as soon as the meeting began Friday, declaring that its sensitive uranium enrichment work was not up for debate.

Speaking shortly after Ashton, its chief negotiator Saeed Jalili insisted that Iran "has the right to a combustion cycle, including the enrichment of uranium," under the nuclear Non-Proliferation Treaty.

"This right must be recognised," he said, stressing that "we are ready for talks, even tomorrow" if the six powers were to accede to the long-standing Iranian demand.

Western sources familiar with the talks said the Iranians also insisted that international sanctions against Tehran be lifted as a precondition for talks on a possible nuclear fuel swap.

The Istanbul gathering was the second between the two sides after talks resumed last month in Geneva, breaking a 14-month hiatus in diplomatic efforts to settle the dispute.

Ashton stressed that while the Iranian "preconditions" blocked progress, the negotiating powers were committed to diplomacy.

The P5+1, she said, sought to negotiate a revised version of the nuclear fuel swap proposal and ways to improve transparency through monitoring measures by the UN atomic watchdog.

The powers believe the swap scheme, first discussed in 2009, could ease suspicions over Tehran's activities, but say its terms should be modified in line with Iran's growing stockpile of low-enriched uranium (LEU).

Under the original draft, Iran would have received fuel for a Tehran medical research reactor from France and Russia in return for shipping out 1,200 kilogrammes of LEU, or most of its stockpile at the time.

After a prolonged stalemate, Brazil and Turkey brokered a modified deal with Iran in May.

But the United States rejected that accord, arguing it had failed to take into account additional uranium Iran enriched in the meantime, and led the UN Security Council in imposing a fourth package of sanctions.

A viable modification of the terms would require a "great increase" in the quantity of LEU Iran should ship out, a Western official said in Istanbul.

The US diplomat commented that Iran had tried and failed to split the P5+1 group, hoping to extract concessions,.

"If that was their calculation, they miscalculated because I have seen a 5+1 group more united as ever before," he said.

Iran insists its nuclear programme is peaceful, but has refused to suspend uranium enrichment, the sensitive process which can be used to make nuclear fuel or, in highly extended form, the fissile core of an atomic bomb.

Its defiance has prompted four sets of UN sanctions, coupled by a series of sanctions imposed unilaterally by the United States and the EU.

Russia is going through the final motions of ratifying a new START treaty that reduces old nuclear warhead ceilings by 30 percent and limits each side to 700 deployed long-range missiles and heavy bombers.

The pact will be submitted for a last vote to Russia's lower house of parliament on January 25 and almost certainly be ratified by the upper chamber the following day.

It was backed by the US Senate last month.

But analysts said that Moscow and Washington have little time to rejoice having put in motion the first round of mandated nuclear weapons reductions since the Cold War.

Obama, who pledged to "reset" Russia-US relations, sees START as only a stepping stone to further cutbacks, but a top Russian official made clear last week that the president's insistence for another round of negotiations later this year was not being received well in Moscow.

"I am convinced that before talking about any further steps in the sphere of nuclear disarmament ... it is necessary to fulfill the new START agreement," Foreign Minister Sergei Lavrov told reporters.

Only "then will be it be clear what additional steps should be taken to strengthen global security," he added.

In Russia's view, the round of disarmament which covers short-range tactical missiles dear to Moscow, balances out the West's current dominance in modern conventional forces.

The US Bulletin of the Atomic Scientists estimates that Russia has 2,050 deployed tactical warheads that could be deployed in small nuclear campaigns in its periphery. The United States has just 500.

Lavrov said Russia's commitment under the new START treaty will not be "fulfilled" for another seven years, and some analysts interpreted the comments as a flat-out rejection of Obama's latest overture.

"It seems Lavrov meant that these talks will not start for another seven years," said independent military commentator Alexander Golts.

"It is also important to note that Lavrov said these talks should be tied to space and conventional weapons," said Golts.

"This is basically a polite way of saying that we are not ready to talk about it."

The Russian foreign minister Lavrov spelled out a series of amendments that appear inherently unacceptable to the United States.

They include the prohibition of military space programmes that the Pentagon is currently studying and a requirement for all talks to include conventional warheads that the West is developing much faster than Russia.

Lavrov even suggested that the next round of talks should for the first time involve other countries -- presumably China and other emerging nuclear powers that may press their own demands on Washington.

"Most Russian experts see nuclear weapons as an equalizer," said Moscow's Centre for Disarmament Director Anatoly Dyakov.

"They believe that the removal of nuclear weapons must be accompanied by a full transformation of international relations that ensures that no country can suddenly decide to use force."

Military analysts estimate that it will take Russia another decade to develop a conventional weapons programme capable of re-establishing some semblance of parity with the West.

But they warned that it is highly unlikely that Russia will be able to delay the next round of nuclear negotiations for as long as suggested by Lavrov.

"We are going to have to start these whether we like it or not," said Institute for Strategic Assessment head Alexander Konovalov.

"The Americans will not stand for this kind of disparity."

"I think it is part of an attempt to produce nuclear bombs on top of plutonium-based ones," Kim Sung-Hwan said in an interview with Yonhap News Agency published Thursday.

The North raised regional security fears last November by revealing an apparently operational uranium enrichment plant to visiting US experts.

It says the plant will fuel an experimental light water reactor now under construction to generate electricity.

But senior US and other officials fear it could easily be reconfigured to produce weapons-grade uranium to augment the country's existing plutonium stockpile.

The North quit six-party nuclear disarmament talks in April 2009 and conducted its second atomic weapons test a month later.

The forum, chaired by the North's major ally China, also includes the two Koreas, the United States, Japan and Russia.

Kim said efforts to revive the negotiations have recently gained momentum. He said Seoul, Tokyo and Washington have "considerably narrowed down opinions" with Moscow also "getting there to some degree".

"China has slightly different opinions on the North's stance so we have more to discuss (with Beijing)," the minister told Yonhap.

The North has indicated willingness in principle to return.

But the United States, Japan and South Korea say it must mend ties with the South and show seriousness about denuclearisation before discussions resume.

Cross-border ties have been icy since South Korea accused the North of mounting a torpedo attack on a warship, the Cheonan, last March with the loss of 46 lives. The North denies involvement in the sinking.

Tensions rose to their highest level for years when the North on November 23 shelled a South Korean border island, killing four people including civilians.

But in a change of tack this year, Pyongyang has been calling for dialogue.

Kim restated the South's conditions for any dialogue, saying the North must take responsibility for the attacks and show commitment to nuclear disarmament.

"There should be some sort of responsible measures about the Cheonan's sinking and the Yeonpyeong shelling. There should be a firm pledge that such provocations won't be repeated and we also have to first make sure (the North) is serious about denuclearisation," he told Yonhap.