At least 12 people have been killed in air strikes on the northern Syrian city of Raqqa, the de facto capital of the Islamic State terror group, the state news agency and an opposition monitoring group said.

State news agency Sana reported on Tuesday that women and children were among the dead after the strikes by the US-led coalition.

It said 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 air strikes on Monday night, which it said were probably 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.