Heavy clashes between Syrian troops and rebels trying to break a government siege in the suburbs of Damascus have killed at least 160 fighters over two days, activists have said.

Forces loyal to Syrian President Bashar Assad have laid siege for months to rebel strongholds in the Ghouta area east of Damascus, preventing food, clean water, medicine and other supplies from entering in a bid to crush resistance.

The tactic, which activists say has led to famine, has helped government troops capture a string of rebel-held areas over the past month on Damascus' doorstep.

The government push around the capital has coincided with gains by Assad's forces around the northern city of Aleppo as well as a new offensive in the rugged Qalamoun region north of Damascus.

The recent victories have shifted the momentum of the conflict in Assad's favour and given the Syrian leader greater leverage in proposed peace talks that the US and Russia are trying to convene to end the civil war.

The intense fighting in the eastern Ghouta area began on Friday when several rebel groups attacked government forces, according to the British-based Syrian Observatory for Human Rights and an activist based in Qalamoun.

Observatory director Rami Abdurrahman said the rebels were trying to open the road between Ghouta and the outside.

He said more than 160 fighters were killed on Friday and yesterday, including nearly 100 rebels, most of them from al Qaida-linked groups, the Nusra Front and the Islamic State of Iraq and the Levant.

Among more than 60 fighters killed on the government side were 20 gunmen from the Iraqi Shiite Abu al-Fadl al-Abbas brigade, he said.