Syrian government airstrikes, including one on a crowded sheep market, killed at least 11 people on Thursday in a northern town controlled by the Islamic State group, activists said.

After largely ignoring the rise of the Al Qaeda splinter group over the past year, President Bashar Assad's air force has begun to target Islamic State militants more regularly since the extremist faction seized control of much of northern Syria and a large part of neighboring Iraq in June.

Two activist groups, the Aleppo Media Center and the Syrian Observatory for Human Rights, said Thursday's air raids struck the town of al-Bab in Aleppo province.

The Britain-based Observatory said at least 11 people were killed and more than 17 wounded. The AMC put the death toll at 19 and said another 20 people were wounded.

It was not possible to reconcile the difference in casualty figures, but numbers frequently vary in the chaotic aftermath immediately following attacks.