BEIRUT, Lebanon — Syrian government forces continued their bombing campaign in the northern city Aleppo on Saturday, with a single strike in a crowded vegetable market killing at least 21 people, activists and residents said.

Activists in Aleppo said that more than 400 people had been killed in nearly two weeks of airstrikes and barrages of improvised “barrel bombs” packed with explosives that are dropped from low altitudes by helicopters.

Peace talks brokered by Russia and the United States are scheduled to begin Jan. 22 in an attempt to end the Syrian civil war, which began as a protest movement against President Bashar al-Assad. After a government crackdown, it transformed into a conflict that has killed more than 120,000 people.

Many activists contend that the government is increasing the use of force to demoralize the opposition and gain ground before the conference. Others believe that the attacks are meant as revenge for recent insurgent assaults in Adra, north of Damascus, in which many civilians were reported to have been killed.