BEIRUT - A Syrian government air strike 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 al-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. Some observers say the Aleppo assault fits into Assad's apparent strategy of trying to expose the opposition's weakness to strengthen his own hand ahead of the negotiations.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists on the ground, said 25 people, including four children, were killed and dozens were wounded in the strike. The Aleppo Media Center published a list of 21 names of people it said were killed in the air raid.

The discrepancy could not be immediate reconciled, but differing death tolls are common in the aftermath of such attacks.

An activist with the Aleppo Media Center, Hassoun Abu Faisal, said the strike took place around 10 a.m. local time when the market was packed with shoppers. "Cars were damaged, debris and rubble are everywhere," he said via Skype. "Many of the wounded have lost limbs."

Both Abu Faisal and the Observatory reported air strikes in other opposition-held areas of Aleppo, including Myassar, although there was no immediate word on casualties.

Aleppo, Syria's largest city, has been a major front in the country's civil war since rebels launched an offensive there in mid-2012. The city has been heavily damaged since then in fighting that has left it divided into rebel- and government-controlled areas.

Another critical battleground is around the capital, Damascus. Assad's forces have a tight grip on the heart of the city, but many of the suburbs have been opposition strongholds since the early days of the uprising.