Five people, including a child, died from army rocket fire at Daraya, opposition activists said.
Daraya is one of a series of Sunni Muslim suburbs ringing Syria's capital that have been at the forefront of the 21-month-old revolt against President Bashar al Assad.
"This is the biggest attack on Daraya in two months," said Abu Kinan, an activist in the area. "An armoured column is trying to advance but it being held back by the Free Syrian Army."
He said tens of thousands of civilians had fled Daraya during weeks of government assault but 5000 remained, with hundreds of rebels. Daraya is near the main southern highway leading to the Jordanian border 50 miles south.
Activists said the military is trying to push back rebels who have been slowly advancing from the city's outskirts to within striking distance of districts inhabited by Mr Assad's Alawite minority sect.
Mr Assad's forces have relied on air power and artillery. Rebels have been able take several outlying towns and have clashed with government troops near Damascus International Airport, halting flights.
Another activist in Damascus said Daraya has been a firing position for rebel mortars and rockets. From it, they have hit a presidential complex on a hill overlooking Damascus and pro-Assad shabbiha militia nearby.
"I think the regime has realised that it no longer can afford to have such a threat so close by," he said.