ANKARA — A bomb claimed by ISIS at a Syria-Turkey border crossing killed at least 25 people, most of them foreign-backed Syrian rebels, and wounded dozens more on Thursday, witnesses said.

The British-based Syrian Observatory for Human Rights monitoring group put the death toll at least 21.

The attack targeted rebels from factions which have been battling the jihadist group with Turkish military support along another stretch of border further to the northeast, the witnesses and the Observatory said.

It took place on the Syrian side of the Atmeh crossing, west of Aleppo. A photo sent by a witness in the area, purportedly of its aftermath, showed the bloodied corpses lying on the ground.

ISIS claimed the attack in an online statement.

Syrian rebels are expecting stiff resistance from ISIS near a northern Syrian village that is of great symbolic significance to the group, a rebel commander said on Wednesday, signaling a tough battle ahead to capture it.

With Turkish backing, rebels fighting under the Free Syrian Army banner have advanced to within a few miles of the village of Dabiq, the site of an apocalyptic prophesy central to the militant group’s ideology.

The rebels this week captured the village of Turkman Bareh, 4 miles east of Dabiq, after heavy clashes – part of a wider Turkish-backed operation to clear ISIS from the border area.