At least 22 people, mainly children, have been killed in airstrikes outside a school in northern Syria, say Syrian activists.

The Idlib News network said the airstrikes hit as the children gathered outside a school complex in the village of Hass, in northern Idlib, on Wednesday.

The group says most of the casualties are children and said there are fears the death toll could rise further as some of the wounded are in a critical condition. 

The Britain-based Syrian Observatory for Human Rights put the death toll at 22, including 14 children. It also says the death toll is likely to rise.

Idlib is the main Syrian opposition stronghold, though radical groups have a large presence there also. It has been regularly hit by Syrian and Russian warplanes as well as the U.S.-led coalition targeting Islamic State militants.