Syrian opposition activist groups say airstrikes have killed at least 17 people just south of the northern city of Raqqa, the de facto capital of the militant Islamic State (IS) group.

The Britain-based Syrian Observatory for Human Rights said 18 people were killed in the airstrikes on the road between the villages of Ratla and Kasrat. It said Sunday's airstrikes hit busses, adding that the identity of the dead is not known.

The activist-operated Raqqa is Being Slaughtered Silently said 17 civilians were killed in the airstrike on busses carrying civilians.

Both groups blamed the US-led coalition, which has been carrying out airstrikes in Syria against IS since September 2014.

US-backed Syrian fighters have been marching toward Raqqa for weeks under the cover of coalition airstrikes.