North and South Korean soldiers traded fire Thursday at the heavily fortified frontier that divides the two rival nations. The shooting comes as the United States, China and other nations work toward resolving an escalating crisis over the communist state's nuclear ambitions.

South Korean military spokesman Colonel Lee Hong-ki briefed reporters on the machine-gun shut out in the Demilitarized Zone, or DMZ, a buffer area between North and South Korea intended to keep their two armies apart.

He says the incident took place at 0600 Thursday local time along the DMZ, with North Korea firing four rounds first at a South Korean post. He says the South answered with a warning broadcast, then returned fire with 17 rounds.

The South Korean military says none of its soldiers were injured in the skirmish. Such shooting incidents in the DMZ are rare, with the last occurring in 2001.

Officials from the U.S.-led United Nations Command, which oversees the southern half of the DMZ, inspected the site of the shooting near the South Korean town of Yonchon, 60 kilometers north of Seoul and are still investigating what happened. But regardless of the cause, there are concerns an event like this will increase tensions on the Korean peninsula - tensions that already are high due to North Korea's nuclear activities.

The United States, China, Japan and other nations are now trying to convince Pyongyang to agree to multilateral negotiations on halting its nuclear weapons program, which violates international agreements.

But so far the North is insisting on direct talks with the United States, which it blames for the dispute that flared last October after U.S. officials say North Korea admitted running a secret nuclear program despite a 1994 disarmament deal with Washington.

Japanese Foreign Minister Yoriko Kawaguchi said Thursday that it is essential for North Korea's Asian neighbors to search for a peaceful end to the nuclear stand-off. She says Japan will continue to push North Korea to agree to multilateral talks.

The communist state heightened regional worries last week when it said that it had reprocessed enough spent reactor fuel to build several nuclear weapons, a claim the Bush administration is trying to verify. It has warned it has nuclear arms and if provoked, will use them against the United States.

Despite the threat, U.S. Secretary of State Colin Powell on Wednesday expressed optimism about diplomatic efforts, saying he expected to see positive developments soon.

He was speaking after a phone conversation with Chinese Foreign Minister Li Zhaoxing in which the two discussed North Korea. Earlier this week, Beijing dispatched an envoy to Pyongyang in an effort to break the impasse and broker an agreement on talks between the United States and North Korea. China is working toward a regional forum that would allow direct contact between U.S. and North Korean officials.