Syrian troops pounded a flashpoint district in southern Damascus on Sunday, a watchdog said, as violence raged in several rebel-held towns in the suburbs of the capital.

The Syrian Observatory for Human Rights said the army shelled the district of Assali, as well as the battered towns of Irbin and Harasta east of Damascus.

Near Harasta, the Britain-based monitoring group added, there were sporadic clashes between rebels and troops waging an intense campaign to try to reclaim an eight-kilometre (five-mile) radius around the capital.

Irbin and Harasta are in the Eastern Ghuta area of Damascus province, home to some of the rebel Free Syrian Army's fiercest and best organised groups.

Southwest of the capital, tanks shelled the town of Daraya, which the army has been trying to retake for almost a month, activists said.

"Tanks renewed shelling the town of Daraya, while warplanes overflew the area," said the Syrian Revolution General Commission, a grassroots network of anti-regime activists.

Elsewhere, rebels clashed with troops near an important military academy in Mulsimiyeh just north of the embattled city of Aleppo, said the Observatory.

Clashes raged "after the army deployed reinforcements" to the area a day after rebels captured two thirds of the sprawling complex, the monitoring group added.

More than 43,000 people have been killed since the uprising against President Bashar al-Assad's rule erupted in March 2011, according to the Observatory which relies on a network of activists and medics on the ground in compiling its tolls.