Syria's state news agency and an opposition monitoring group say air strikes on the northern city of Raqqa, the de facto capital of the Islamic State group, have killed at least 12 people.

State news agency SANA reported on Tuesday that the air strikes by the US-led coalition killed 12, including women and children. It says the families were fleeing the city in boats across the Euphrates River ahead of an expected all-out attack by the Kurdish-led Syrian Democratic Forces.

The Britain-based Syrian Observatory for Human Rights said 21 people were killed in the Monday night air strikes. It said they were likely carried out by the US-led coalition.

The air strikes have intensified in recent weeks as SDF fighters have reached the outskirts of the city.