ALEPPO, Syria - Syrian army tanks shelled Aleppo on Sunday and a helicopter gunship strafed rebel positions with heavy machinegun fire as they fought for control of the country's biggest city and key battleground of the 17-month uprising.After UN Security Council paralysis on Syria forced peace envoy Kofi Annan to resign last week, and with his ceasefire plan a distant memory, rebels have been battered by the government onslaught in Aleppo and the capital Damascus.A Reuters correspondent in Aleppo witnessed fierce street fighting in the Salaheddine district, a gateway into the city of 2.5 million people.Tanks pounded alleyways where rebels sought cover and one shell hit a building next to the reporter, pouring rubble onto the street and sending huge billows of smoke into the sky.