Opposition fighters battled Syrian government forces in Damascus into the early hours of Monday in what residents described as the fiercest fighting yet inside the capital.
Activists said the fighting spread from the south of the city to a second area as night fell. At least five people were killed and dozens wounded, locals said.
The spread of fighting came as U.N. peace mediator Kofi Annan was due to fly to Moscow for a two-day visit in which he will meet Russian President Vladimir Putin who has resisted Western calls to increase pressure on Syrian leader Bashar al-Assad.
Numerous Damascus residents contacted by Reuters said they could hear loud explosions, persistent gunfire and sirens wailing overnight, and described the fighting as the worst so far of the 17-month uprising against Assad.
Thick black smoke was visible above the Damascus skyline in live Internet video links. Government troops closed the airport road, activists said.
"I can't believe it, it sounds incredibly close. I hear shooting and other stuff, like blasts. I can hear the sounds of ambulances rushing past. I am so afraid. People may die tonight," said a resident contacted by telephone in a district close to the fighting.
Activist Samir al-Shami, who spoke to Reuters by Skype from Damascus, said the fighting was under way in the al-Tadamon district in the capital's south, after sustained battles began at nightfall on Saturday in the nearby Hajar al-Aswad district.
"There is the sound of heavy gunfire. And there is smoke rising from the area. There are already some wounded and residents are trying to flee the area," he said, using Skype to show live video images of smoke visible over the skyline.
"There are also armored vehicles heading towards the southern part of the neighborhood," he said.
Another activist reached by Skype said the fighting later spread to al-Lawan, a neighborhood on the southwestern outskirts of the capital.
A third activist, who also asked not to be identified, said: "We've been expecting things to worsen in Damascus after the army crushed the rebellions in some of the suburbs, like Douma outside the capital. There were thousands of fighters in some of those suburbs. Some of them were killed but a lot of them fled and they've been heading to the capital itself."
Some residents said the fighting eased slightly as the night wore on but many protests in support of the opposition were staged in the poorer neighborhoods of Damascus.
One local, who gave his name as Tarek, said residents set tires ablaze to distract the security forces and relieve pressure on the fighters in Tadamon.
The government restricts access to the country by independent media, making verification of events difficult.
Annan is travelling to Moscow days after opposition reports of a new massacre prompted a fresh wave of denunciations in the West where there are hopes Putin might ease his support for Assad. But Moscow has shown no public sign of wavering in its backing for its last major Arab ally, a customer for its arms and host to a Russian naval support base.
Along with China, Russia has blocked tougher U.N. Security Council action and the West has shown no appetite for the kind of intervention it undertook last year when NATO helped topple Libya's Muammar Gaddafi.
Annan said on Friday he was "shocked and appalled" at the government for breaking a promise not to use heavy weapons in populated areas, and that it was confirmed that helicopters and artillery had fired on the village of Tremseh.
Syrian Foreign Ministry spokesman Jihad Makdissi criticized Annan for jumping to conclusions by accepting opposition reports of the incident last week.
"What happened was not a massacre ... what happened was a military operation," Makdissi told a news conference in Damascus. "Government forces did not use planes, or helicopters, or tanks or artillery."
U.N. observers returned to the village on Sunday to gather more evidence at the site after finding signs that artillery was used but inconclusive evidence of the scale of the killing.
The Syrian government said it killed several dozen enemy fighters in battle in Tremseh but denied accusations that it carried out a massacre or that its forces used heavy weapons.
Opposition footage of the incident on the Internet has shown bloody corpses of men, but not women or children, making it difficult to determine whether those killed were fighters.
Sander van Hoorn, a Dutch journalist who reached Tremseh, said by Twitter that he had counted 30 graves in the town and had seen clear evidence of shelling, including of a school used as a shelter by refugees.
He said the evidence on the ground clearly contradicted the government's assertion that no heavy weapons were used. But he also said he had not yet seen signs of a massacre like one that took place in the city of Houla in May, when the United Nations says 34 women and 49 children were among 108 people killed.
(Additional reporting by Marwam Makdesi in Damascus and Yeganeh Torbati in Dubai.; Writing by Peter Graff and Robin Pomeroy; editing by Christopher Wilson)