Syrian gunships pounded rebels and troops clashed with insurgents in Aleppo as world powers voiced fears over an all-out assault and called for "maximum pressure" to prevent a massacre.

Columns of armed forces and armour have poured into Aleppo over the past two days, with troops firing on a string of rebel neighbourhoods in the battle for control of the pivotal northern city.

Amid widespread expectations that an increasingly desperate President Bashar al-Assad will pull out all the stops to secure Aleppo, residents of some areas of the commercial capital fled in fear of a massacre.