Syrian activists say at least 17 people, most of them fighters, have been killed in a car bomb blast at a rebel checkpoint near the Turkish border.

The Syrian Observatory for Human Rights monitoring group said it could confirm 14 militants were killed in the attack on the road connecting the rebel-held town of Azaz to the Bab al-Salameh border crossing. It is one of the more important junctions between Syria's rebels and their Turkish backers.

A joint Russian-government offensive pushed rebels away from the contested city of Aleppo and into Azaz earlier this year, where they are hemmed in by Islamic State extremists to the east and Syrian Kurdish forces to the west. No side has claimed responsibility for the blast.

The Local Coordination Committees activist network put the toll at 30 fatalities.