At least 29 Turkish-backed Syrian rebels were killed in a blast claimed by ISIS at a border crossing with Turkey on Thursday, the Syrian Observatory for Human Rights and witnesses 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.

But the ISIS-linked Amaq news agency said a suicide car bomber struck a convoy of rebels.

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

A photo sent by a witness in the area, purportedly of its aftermath, showed the bloodied corpses lying on the ground.

Rebel fighters have been targeted at the crossing before. ISIS claimed responsibility for a mid-August suicide attack there that killed at least 32.

Most of the rebels in the Atme blast were from a group called Failaq al-Sham, witnesses said. The rebels were among those participating in Turkey’s operation dubbed “Euphrates Shield” in neighboring Aleppo province.

Among the dead were the head of the top civil judicial body in rebel-held eastern Aleppo, Sheikh Khaled al-Sayyed, and a judge who worked with him, a witness and a rebel official said.

Ankara began the unprecedented cross-border offensive 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 Jarablus and is pushing towards the jihadist-held towns of Dabiq and Al-Bab.

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