Rebel fighters search men who fled from ISIS-controlled areas, upon their arrival in the northern Syrian rebel-held town of Waqf, near al-Rai town, in Aleppo Governorate, Syria, September 29, 2016.

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 explosion at the Atme crossing between Turkey and the northern Syrian province of Idlib, adding that around 20 people had also been wounded.

ISIS, which has been fighting the rebels and their Turkish allies, said a “soldier of the caliphate” detonated a car bomb as the rebel fighters were crossing into Syria, US-based monitor SITE Intelligence Group reported.

The ISIS statement said several commanders of the Islamist Ahrar al-Sham rebel group were among the dead.

Turkey’s state-run Anatolia news agency said the deadly blast took place during a “change of guard” among Syrian rebels in the area.

Rebel fighters have been targeted at the crossing before. ISIS 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 neighboring Aleppo province.

Ankara began the unprecedented cross-border operation on August 24, saying it was targeting both ISIS and the Kurdish YPG militia which Turkey considers a “terrorist” group.

The operation has so far captured the ISIS stronghold of Jarabulus and is pushing towards the militant-held towns of Dabiq and Al-Bab.

Dabiq holds symbolic importance for ISIS because of a Sunni prophecy that states it will be the site of an end-of-times battle between Christian forces and Muslims.

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