BEIRUT — At least 21 people were killed in an airstrike on a market in northern Syria, activists reported Monday.

The Britain-based Syrian Observatory for Human Rights said there were three strikes on a market in Atareb in the opposition-held countryside outside Aleppo on Monday. Aleppo, Syria’s largest city, is controlled by the government.

Atareb has been hit by Russian and Syrian government airstrikes since 2015, when Russia intervened in the war in Syria to turn the tide in President Bashar Assad’s favor. The town has become home to tens of thousands of people displaced by fighting in nearby areas.

Atareb and the opposition-held countryside in northwest Syria is supposed to be protected by a ‘‘de-escalation agreement’’ brokered earlier this year by Russia, Iran, and Turkey, the main backers to the Syrian government and the opposition. The agreement is regularly violated.

The United States and Russia have lately renewed efforts to find a settlement for Syria, where at least 400,000 people have been killed and 11 million displaced in a six-year long civil war.