Helicopter gunships have opened fire over Aleppo on Sunday with artillery booming across the city as government forces and rebels fight for control.

Clashes have been reported in several rebel-held districts in what could herald the start of a decisive phase in the battle for Aleppo, Syria's second city.

The situation has led a Syrian opposition leader to urge foreign allies to circumvent the UN 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.

Assad's forces, including tank columns, have been massing around Aleppo for an assault to take back districts back from rebel units. Skirmishing has been going on for days in areas such as southwestern Saleheddine.