Residents of Douma, 15 km (10 miles) from Damascus, said hundreds of people were fleeing the town as government forces swept the streets in search of rebels trying to overthrow President Bashar al-Assad. They reported many bodies buried under the rubble of houses in the town of half a million.
It was not possible to immediately verify the reports on the day world powers met in Geneva to try and find a way to resolve the increasingly bloody conflict in Syria.
Abdullah, 50, said he and his five children left Douma on Saturday morning. "I saw at least three bodies on a street corner, some houses were destroyed, others were on fire. Only a few people remained inside the city. Those who can, leave," he said by telephone from a nearby town.
"I saw a body on the side of the street and dogs were gathering around it, it was really horrible... we are all living as refugees inside our country," he added.
Abdullah left in a convoy of 200 people from his neighbourhood. He said eight people were packed into each car. They passed through four checkpoints where soldiers in "full gear" eyed their identity cards before allowing them through.
Syrian forces have used similar tactics countrywide, activists say, of long sieges and shelling before raids.
A 16-month uprising against Assad has morphed into a fight between rebels and the government which labels armed and unarmed dissidents as "terrorists."
Other residents reported hundreds of people fleeing Douma.
Abo Omar, another resident, said rebels withdrew from Douma on Friday night after an intensive 10-day government offensive. "How can the rebels continue fighting with limited guns, facing tanks and helicopters? Now Assad's army is in control of the town," he said, adding that hundreds had been arrested.
"We are calling on the Red Cross and the (Syrian) Red Crescent to go inside the town and remove the bodies. Some of the bodies have started to decay," said Abo Omar. He said many of the dead were buried under the rubble of houses.
Mahmoud Doumany, an opposition activist who also left the town said: "There is no electricity or water, no communication. The situation was really horrible." "Security forces are going into hospitals and we fear for the lives of the wounded," he added.
Syrian forces also on Saturday fired mortar bombs at major cities, activists said. Live stream video footage posted by opposition activists in the eastern desert city of Deir al-Zor showed smoke rising from apartment blocks as continuous explosions rang out.
Activists also reported shelling in Homs, Idlib and Deraa.
The Syrian Observatory for Human Rights said that government shelling and heavy clashes between the army and members of the rebel Free Syrian Army have killed 16 people so far on Saturday.
Opposition activists accuse the international community of inaction. Diplomacy has failed to produce an agreement between Western powers, who favour the opposition, and Russia, which has used its veto power at the United Nations Security Council to block Western and Arab moves to drive Assad from power.