A bomb blast in northern Syria near the border with Turkey killed at least 16 Turkish-backed rebel fighters and wounded others on Thursday, the Syrian Observatory for Human Rights reported.
Two witnesses in the area separately told Reuters the blast killed at least 20 people and wounded many more.
The British-based Observatory said it was not clear if the blast near the Atmeh border crossing was from a suicide attack or a bomb placed in the area. There was no immediate claim of responsibility.
It targeted rebels who have been backed by Ankara in its operation against both Islamic State and Kurdish militants further to the northeast, along a separate stretch of border.