BEIRUT (AP) — Syrian government helicopters and warplanes unleashed a wave of airstrikes on more than a dozen opposition-held neighborhoods in the northern city of Aleppo on Sunday, dropping missiles and crude barrel bombs in a ferocious attack that killed at least 36 people, including 17 children, activists said.

Aleppo has been a key battleground in Syria’s civil war since rebels swept into the city in mid-2012 and wrested most of the eastern and southern neighborhoods from the government. Since then, the fighting has settled into a bloody grind, with neither side capable of mounting an offensive that would expel its opponents from the city.