Syrian government forces kept up a day-long bombardment of rebel-held areas in Aleppo on Saturday.
New pictures uploaded to the internet are said to show rebels firing at army positions. The content cannot be verified.
Opposition activists reported helicopter gunship attacks on the central district of Salaheddine, with tanks and armoured carriers heading towards it.
Aleppo is Syria’s most populous city and there are fears of major casualties.
As in Damascus before it, the battle for control is seen as a crucial test for the regime.
Despite the superior firepower of government forces, neither side has gained the upper hand in Aleppo.
Amid ongoing speculation over President Bashar al-Assad’s future, Russia has strongly denied offering to grant him asylum.
But Foreign Minister Sergei Lavrov warned that Western support for rebels would lead to more bloodshed.