Activists say Syrian government air raids have killed at least 21 people in a northern town controlled by the Islamic State group.

The Aleppo Media Center activist collective and the Britain-based Syrian Observatory for Human Rights both reported the overnight airstrikes on the town of al-Bab in Aleppo province.

Observatory director Rami Abdurrahman said Sunday there were 10 strikes in total, including seven so-called barrel bombs dropped from helicopters. He says at least 21 people were killed and more than 100 wounded.

The Aleppo Media Center put the death toll at 30, with 85 wounded.

Differences in casualty figures are common in the chaotic aftermath of attacks in Syria.