The UN special envoy for Syria, Staffan de Mistura, says he is "appalled and shocked" that rebels in Aleppo are targeting civilians in the city.
The "relentless and indiscriminate" rocket attacks had killed scores of civilians in western Aleppo in the past 48 hours, Mr de Mistura said.
Such attacks could amount to war crimes, he said.
On Friday, rebels began an offensive aimed at breaking the government siege of east Aleppo.
On Sunday, state media in Syria said rebels had used chemical weapons against government-controlled districts of Aleppo.
More than 40 civilians are reported to have been killed in western Aleppo since the rebel attacks began, activists say.
The UK-based Syrian Observatory for Human Rights said 16 were children. It added that 55 soldiers had also died - as well as 64 rebels. source
Armed groups released tens of videos documenting their attacks on the Western side of the city. They used both makeshift homemade weapons they call "hell cannons" and imported weapons likely provided by the U.S. and its allies.