Syrian aircraft and artillery pounded rebel-held areas of Aleppo early Friday as Damascus and its ally Moscow intensified a renewed offensive against the rebels.
It was the fourth straight day of bombing since Russia declared an end to a month-long humanitarian pause on Tuesday.
The more than 250,000 civilians living under siege in the rebel-held east of Syria's second city spent the night in basements and bomb shelters after one of the most intense bombardments of the renewed offensive.
Barrel bombs and other ordnance rained down until midnight only to resume in the early hours, an AFP correspondent reported.
The bombardment came as government troops pressed an assault on the southern neighbourhood of Sheikh Said, which they briefly entered before being pushed back by rebel fighters, the Syrian Observatory for Human Rights said.
The rebels riposted by firing more than a dozen rockets into government-held areas of the city, the Britain-based monitoring group added.
At least 65 civilians have been killed since the offensive resumed on Tuesday, the Observatory said.
Once Syria's main commercial and industrial hub, Aleppo has been devastated by fighting since the rebels overran the east of the city in 2012.
The rebels have been besieged by the army since July and aid agencies say food stocks in the sector are virtually exhausted.
The 1.2mn civilians living in government-held areas have come under repeated rocket fire by the rebels.