BEIRUT — 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, firing missiles and dropping 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.

But over the past two months, President Bashar Assad’s air force has ramped up its aerial campaign on rebel-held areas of Aleppo, pounding them with barrel bombs – containers packed with explosives, fuel and scraps of metal – that cause massive damage on impact.

On Sunday alone, Syrian military aircraft targeted 15 opposition-controlled neighborhoods, said an activist who goes by the name of Abu al-Hassan Marea. The Britain-based Syrian Observatory for Human Rights monitoring group said the Tariq al-Bab district on the eastern edge of the city was hardest-hit.