Syrian rebels battled President Bashar al-Assad's forces near the main intelligence base in the northern city of Aleppo on Sunday as helicopter gunships bombarded several districts of Damascus in an effort to drive out insurgents, witnesses said.

Fighting raged in other parts of Aleppo, Syria's main commercial and industrial hub, and demonstrators defaced overnight a statue of Assad's father, the late president Hafez al-Assad, in the central Shahba area, breaking off parts of the stone edifice, according to a video taken by activists.

"Sounds of explosions from different areas are shaking the whole city. A heavy exchange of gunfire has been going on near the State Security Headquarters in al-Mouhafaza (district) since the morning," a housewife there told Reuters by telephone.

Opposition sources said fighters from rural areas around Aleppo had been converging on the city of three million people near the border with Turkey. The rebel Tawhid Battalion said in a video statement that a battle to "liberate Aleppo" had begun.

In the capital Damascus, Assad's forces appeared to be clawing back territory taken by insurgents earlier in the week, driving them out of the Mezze district, according to residents and opposition activists.

Elite Fourth Division troops were besieging the northern Damascus neighbourhood of Barzeh and the sound of tank fire was heard in the district, they said. Helicopter gunships fired machineguns at the nearby district of Rukn al-Din and Qaboun.