Syrian activists say at least 28 people were killed in Syrian army airstrikes on a town controlled by the Islamic State group in northern Syria.

The Britain-based Syrian Observatory for Human Rights said 19 civilians were among the dead in Saturday's attack on the town of al-Bab, including three children and three women. Scores of others were wounded in the attack which saw army helicopters drop explosives-filled barrels on the town in the northern province of Aleppo.

The Local Coordination Committees group said 29 people were killed.

Al-Bab is held by the Islamic State group and is a frequent target of Syrian army strikes that often kill civilians.

On May 31, Syrian army airstrikes that hit a packed market in al-Bab killed around 70 people, most of them civilians.