At least 29 people were killed and dozens wounded in a suicide bombing that targeted a Syria-Turkey border crossing on Thursday.
According to the Syrian Observatory for Human Rights, the casualties included fighters from the Free Syrian Army (FSA), the Turkish-backed force which has played a pivotal role in the fight against DAESH along the Syria-Turkey border.
The attack took place on the Syrian side of the Atmeh crossing, west of Aleppo, during a change of guard among the FSA, the Observatory said.
According to Anadolu Agency, the blast targeted the Atmeh refugee camp.
In an online statement, DAESH has claimed responsibilty for the attack.
Witnesses said most of the fighters killed in the attack were from the Failaq al-Sham. The group has been fighting alongside other Turkish-backed factions in Ankara's offensive, dubbed "Euphrates Shield", against DAESH.
The fighters use the Atmeh crossing to move between the Syrian province of Idlib, through Turkey, to the areas where anti-DAESH operations are taking place, the Observatory said.
Residents said they also use it to evacuate wounded fighters.
Among the dead from the Atmeh blast 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.
Idlib province, where Atmeh is located, is a bastion of the opposition groups fighting Bashar al-Assad's regime. The multi-sided civil war has raged on for more than five years.
Opposition fighters have been targeted at the crossing before. DAESH claimed responsibility for a mid-August suicide attack there that killed at least 32.
The fighters killed on Thursday were also among those participating in Euphrates Shield.
Ankara began the unprecedented cross-border operation on August 24, saying it was targeting both DAESH and the YPG which Turkey considers a terrorist group.
The operation has so far captured the DAESH stronghold of Jarablus and is pushing towards the towns of Dabiq and Al-Bab.