At least 29 Syrian rebels were killed in a blast at a border crossing with Turkey on Thursday, the Syrian Observatory for Human Rights said.
The Britain-based monitoring group said it was unclear what caused the blast at the Atme crossing between Turkey and the northern Syrian province of Idlib, adding that around 20 people had also been wounded.
Turkey’s state-run Anatolia news agency confirmed a deadly explosion had hit Atme, adding that it took place during a “change of guard” among Syrian rebels in the area.
The IS-linked Amaq news agency also reported the blast, saying it was a car bomb, without carrying any formal claim of responsibility.
Rebel fighters have been targeted at the crossing before. The Islamic State group claimed responsibility for a mid-August suicide attack there that killed at least 32.
The rebels killed on Thursday were among those participating in Turkey’s Operation Euphrates Shield in neighbouring Aleppo province.
Ankara began the unprecedented cross-border operation on August 24, saying it was targeting both IS and the Kurdish YPG militia which Turkey considers a “terrorist” group.
The operation has so far captured the IS stronghold of Jarabulus and is pushing towards the jihadist-held towns of Dabiq and Al-Bab.
Dabiq holds symbolic importance for IS because of a Sunni prophecy that states it will be the site of an end-of-times battle between Christian forces and Muslims.