Syrian state television said in a news flash a helicopter had crashed in Damascus, but offered no further details.

Witnesses said the helicopter was engulfed in flames after it was hit by a projectile during a burst of gunfire and then came down in the nearby district of Qaboun.

Video footage from activists showed the burning helicopter falling to the ground in a trail of smoke, as rebels on the ground could be heard shouting "God is greatest".

The helicopter crashed in a narrow residential street in the northeastern neighbourhood of Qaboun, which is on the outskirts of the capital.

"It was flying overhead the eastern part of the city and firing all morning. The rebels had been trying to hit for about an hour, finally they did," said an activist in the area, who calls himself Abu Bakr and spoke to Reuters by Skype.

Heavy clashes broke out in the eastern outskirts of the capital after the downing of the helicopter, activists said.

Army helicopters began firing rockets and machineguns on Sunday at Jobar, Zamalka and Irbin, working class Sunni Muslim neighbourhoods on the eastern outskirts of the city.

Rebels in the area have been launching guerrilla attacks against Assad's forces on the outskirts of Damascus and neighbouring suburbs, bringing the 17-month-old revolt to the seat of government in recent months.

Activists said the recent round of bombardment was sparked by a rebel attack which killed an army sniper and captured another near a roadblock in Jobar, a rundown neighbourhood near a stadium which activists say has been turned into an army base.

"Assad's army retaliated by arbitrarily arresting 100 people in Jobar. Helicopters dropped fliers warning residents to hand over what the regime describes as terrorists or face annihilation," said Abu Omar, a merchant who lives in the area told Reuters by phone.