Syrian combat aircraft and artillery pounded Aleppo late into the night as the army battled for control of the country’s biggest city, where rebel fighters said troops loyal to President Bashar al-Assad 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.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.The battle for Aleppo, Syria’s largest city, has become a crucial test for both sides in the 16-month-old rebellion. Neither Assad’s forces nor the rag-tag rebels can afford to lose if they hope to prevail in the wider struggle for Syria.Syria’s civil war has entered a far more violent phase since July 18 when a bomb killed four top members of President Bashar al-Assad’s inner circle. Serious fighting reached Aleppo over the past week and rebels also launched an assault on the capital Damascus in July but were repulsed.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.