BEIRUT — A Syrian government airstrike hit a crowded vegetable market in a rebel-held neighborhood of the northern city of Aleppo on Saturday, shattering cars and storefronts and killing at least 21 people, activists said.

For nearly two weeks, President Bashar Assad’s warplanes and helicopters have pounded opposition-controlled areas of the divided city. Activists say the aerial assault has killed more than 400 people since it began Dec. 15.

The campaign comes in the run-up to an international peace conference scheduled to start Jan. 22 in Switzerland to try to find a political solution to Syria’s civil war.