UPDATED: Fiercest fighting yet reported inside Damascus

Opposition fighters battled Syrian government forces in Damascus on Sunday in what residents described as the fiercest fighting yet inside the city limits of the capital

As night fell, activists said the fighting was spreading from the south of the city to a second area, and government troops had closed the airport road.
Numerous residents contacted by Reuters said they could hear loud explosions, persistent gunfire and sirens wailing, and described the fighting as the worst so far of the 17-month uprising against President Bashar al-Assad.
Thick black smoke was visible above the Damascus skyline in live Internet video links.
"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 in a district close to the fighting, contacted by telephone.
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.
"There are hundreds of fighters in Damascus right now, we'll see what happens," said the second activist, who asked not to be named. "If the regime is able to crush the fighters in Tadamon the clashes should stop, but if not they may spread further."
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."
A fourth activist, who gave his name as Tarek, said protests had broken out in other areas of the capital, with residents setting tires ablaze to distract the security forces and relieve pressure on the fighters in Tadamon.
Earlier in the day, an explosion hit a security forces bus in Damascus and wounded several people, activists said. Residents said they heard a powerful blast, followed by sirens of ambulances headed toward Damascus's southern ring road.
Fighting has reached the outskirts of the capital in recent weeks, focusing on poorer areas where anger against the authorities is highest. The past week saw mortars fired inside the capital for what appeared to be the first time.
By afternoon, the opposition Syrian Observatory for Human Rights, which compiles reports by anti-Assad activists, said Sunday's death toll across the country was at least 80. Rockets were being used in the fighting in Damascus, it said.
"The sounds of explosions and heavy gunfire are still heard in the Tadamon neighborhood, the Saidi Qadad area and al-Qazaz in Damascus," it said, referring to neighborhoods in the south of the city.
"There are violent clashes between regime forces and fighters from rebel battalions in these areas. Ambulances were seen transporting members of regime forces out. These clashes are the most violent to happen inside the capital Damascus."
The government restricts access to the country by independent media, making verification of events difficult.
"MASSACRE" DISPUTED
Western countries, Arab neighbors and Turkey have formed an alliance against Assad. But diplomacy has had little impact so far, with Assad's allies Russia and China blocking action by the U.N. Security Council and the West showing no appetite for the kind of intervention it undertook last year when NATO helped topple Libya's Muammar Gaddafi.
Opposition reports of a massacre last week in the village of Tremseh brought a wave of new denunciations of Assad in the West. U.N. observers returned on Sunday to the village to gather more evidence at the site after finding signs that artillery was used but inconclusive evidence of the scale of the killing.
The government says it killed several dozen enemy fighters in battle in Tremseh but denies 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.
Syrian Foreign Ministry spokesman Jihad Makdissi criticized U.N. peace mediator Kofi 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."
Annan had 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 in Tremseh.
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 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.
"Impossible now to say how many civilians got killed in ‪#Tremseh‬ but at first sight (!!) it does not appear to be another Houla," he tweeted.
Annan is due to fly to Moscow on Monday for a two-day visit in which he will meet President Vladimir Putin.
Western countries have repeatedly suggested they see Putin easing 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.
(Additional reporting by Marwam Makdesi in Damascus and Yeganeh Torbati in Dubai; Writing by Peter Graff; Editing by Giles Elgood and Robin Pomeroy)