Clashes in Damascus between state forces and rebels have spread to a second district.
The fighting has been described as the heaviest to reach the capital since the start of the 15-month uprising against President Bashar al-Assad.
Activists in the capital said the Syrian army had closed down the road to the airport and was trying to surround fighters in southern neighbourhoods in an attempt to crush unrest inside Damascus.
The fighting spread to al-Lawan, a neighbourhood on the southwestern outskirts of the capital.
"There are hundreds of fighters in Damascus right now, we'll see what happens," said an activist in the capital.
"If the regime is able to crush the fighters in Tadamon the clashes should stop, but if not they may spread further."
Reports from Syria cannot be independently verified due to restrictions placed on journalists there.
Syria has denied accusations by special envoy Kofi Annan that state forces used heavy weapons or helicopters in clashes in the village of Tremseh last week, where activists said there was a massacre of over 100 people.
Jihad Makdissi, spokesman for Syria's Foreign Ministry, said security forces killed 37 fighters and two civilians in a campaign against the town.
The government has said rebels were launching attacks on other areas from the town.
"Government forces did not use planes, or helicopters, or tanks or artillery. The heaviest weapon used was an RPG (rocket propelled grenade)," Mr Makdissi told reporters at a news conference in Damascus.
"Yesterday we received a letter from Mr Kofi Annan addressed to the Foreign Minister Walid al-Moualem. The least that can be said about this letter about what happened in Tremseh is that it did not rely on facts."
Russian President Vladimir Putin will meet UN special envoy Kofi Annan in Moscow on Tuesday to discuss the crisis in Syria.
Mr Annan is expected to increase international pressure on Russia to do more to help end the bloodshed in Syria following accusations by the Syrian opposition that Moscow is prolonging the conflict by propping up President Bashar al-Assad.
The Kremlin said Mr Putin would underline Russia's support for Annan's peace plan, which calls for both the government and opposition in Syria to work together to end the crisis.
