Hillary Clinton, the US Secretary of State, said she has "great hope" that the meeting in Geneva of world powers could be a turning point in the crisis.
Mr Annan, she said, "has developed his own very concrete road map for political transition" from the Assad regime.
"We believe it embodies the principles needed for any political transition in Syria that could lead to a peaceful, democratic and representative outcome reflecting the will of the Syrian people."
The UN meanwhile said that the violence in Syria has worsened since a cease-fire deal in April, and the bloodshed appears to be taking on more dangerous, sectarian overtones.
The UN's deputy envoy for Syria, Jean-Marie Guehenno, told the UN Human Rights Council that Annan's six-point peace plan "is clearly not being implemented".
Reflecting the sense of urgency, Mr Annan, the UN and Arab League envoy to Syria, summoned permanent members of the UN Security Council - Britain, China, France, Russia and the United States, plus Turkey - for the conference.
Syrian officials denounced what they called a rebel "massacre against the freedom of the press".
According to the official news agency SANA, the attack on Ikhbariya TV occurred in the town of Drousha, about 14 miles south of the capital Damascus. Hours after the attack, the station was still on the air broadcasting its programs.
Ikhbariya is privately-owned but strongly supports President Bashar Assad's regime. Pro-government journalists have been targeted on several previous occasions during the 15-month uprising, although such incidents are comparatively rare.
Earlier this month, two Ikhbariya employees were shot and seriously wounded by gunmen in the northwestern town of Haffa while covering clashes between government troops and insurgents.
Rebels deny they target the media. The Britain-based Syrian Observatory for Human Rights confirmed the raid and the deaths of several employees, but had no other information.
Information Minister Omran al-Zoebi told reporters outside the station that gunmen stormed the compound, placed explosives and then detonated them.
"What happened today is a massacre, a massacre against the freedom of the press," al-Zoebi said in comments broadcast on state-run Syrian TV. "They carried out a terrifying massacre by executing the employees."
An employee at the station said several other employees were wounded in the attack and other guards were kidnapped when the gunmen attacked just before 4 a.m. local time.
The employee, who did not give his name for fear of repercussions, said the gunmen drove him about 200 meters (yards) away, and then he heard the explosion of the station being demolished. "I was terrified when they blindfolded me and took me away," the man said by telephone.
State-run Syrian TV showed a demolished structure without a ceiling, saying it was the station's main studio. It also showed what it said were tapes on fire amid piles of debris.
