Syrian combat aircraft and artillery pounded Aleppo late into Tuesday night as the army battled for control of the country's biggest city, while rebel fighters said President Bashar al-Assad's forces had been forced to retreat.

During the day on Tuesday large clouds of black smoke rose into the sky after attack helicopters turned their machineguns on eastern districts for the first time in the latest fighting and a MiG warplane later strafed the same area, Reuters reported.

After nightfall, Reuters journalists in Aleppo heard loud explosions somewhere near the city. At least 10 volleys of shells lit up the night sky and drowned out the sound of the Islamic call to prayer. Carloads of rebel fighters shouting "God is great" sped off towards the fighting.

Heavy gunfire echoed around the Salaheddine district in the southwest of the city, scene of some of the worst clashes, with shells raining in for most of the day. Neither side appeared to be in full control of the area.

"The regime has tried for three days to regain Saleheddine, but its attempts have failed and it has suffered heavy losses in human life, weapons and tanks, and it has been forced to withdraw," said Colonel Abdel-Jabbar al-Oqaidi, head of the Joint Military Council, one of several rebel groups in Aleppo.