Helicopter gunships opened fire overAleppo on Sunday and the thud of artillery boomed across neighborhoods as government forces and rebels fought for control of Syria's second city, Reuters reports.

Opposition activists reported clashes in several rebel-held districts in the morning in what could herald the start of a decisive phase in the battle for Aleppo, Syria's commercial hub.

International peace envoy Kofi Annan and other foreign leaders said the situation in Aleppo served to emphasize the need for a negotiated political solution to the conflict in Syria, now in its 16th month.

A Syrian opposition leader urged foreign allies to circumvent the divided U.N. Security Council and intervene directly in the struggle to topple President Bashar al-Assad.

"Our friends and allies will bear responsibility for what is happening in Aleppo if they do not move soon," said Abdelbasset Sida, the head of the Syrian National Council, the main alliance for opposition to Assad.

Assad's forces, including tank columns, have been massing for an assault to wrest back neighborhoods from rebel units and skirmishing has been going on for days in districts such as southwestern Saleheddine.

One opposition activist said he had seen tanks and armored troop carriers heading towards Salaheddine on Sunday.
The Syrian Observatory for Human Rights, a pro-opposition monitoring organization, said fighting was taking place early on Sunday in central districts of the city and a neighborhood on the western outskirts.

Rebel-held areas visited by a Reuters reporter appeared to be deserted by residents. Fighters were using houses - some of them clearly abandoned in a hurry, with food still in the fridges - as their bases.
A jet fighter flew over the city shortly after dawn.