President Bashar al-Assad's forces fired rocket barrages on Friday at a Damascus suburb hit by nerve gas last week, in another attempt to capture the strategic town ahead of a possible U.S. strike, opposition activists said.
Elite guard units backed by tanks advanced from two directions on the suburb of Mouadamiya, 8 km (5 miles) west of Damascus along the road to the nearby Israeli-occupied Golan Heights, but were met with heavy resistance from two rebel brigades dug in the town, they said.
Four rebel fighters were killed, the opposition sources said. There were no immediate reports of casualties among loyalist forces. Restrictions by Syrian authorities on independent media makes verification difficult.
At least 80 people were killed by a poison gas attack on Mouadamiya on the morning of Aug. 21, an hour after hundreds of people died from a similar attack in eastern neighbourhoods of Damascus, according to opposition activists.
The United States made clear on Friday that it would punish Assad for the "brutal and flagrant" chemical attacks that it says killed more than 1,400 people.
Assad has denied using chemical weapons.
His forces have intensified the shelling of Mouadamiya and eastern Damascus neighbourhoods since Aug. 21, hoping to drive out rebel brigades, who had encroached on his seat on power in the capital, according to opposition activists.
Mouadamiya borders the Mezze military Airport, a main base for loyalist troops and militia, and the headquarters of the Fourth Mechanised Division, which is headed by Assad's feared brother Maher and comprised mainly of troops from his Alawite minority sect, an offshoot of Shi'ite Islam, that has dominated Syria for since the 1960s.
Activist Wassim Ahmad said the missile barrages hitting Mouadamiya were the heaviest since the suburb was besieged by the Fourth Division and Republican Guards units nine months ago.
"It appears the regime is trying to seize Mouadamiya to create a distance between the rebels and the Mezze airport and the Fourth Division, before the American strike makes them more of a threat," he said.
Ahmad said thousands of civilians remained besieged in the suburb, along with the fighters. He added that shelling on Thursday killed three members of the one family. A boy and a girl survived after their father, mother a 12-year-old brother were killed, he said.