At least 25 people, mostly Turkish-backed Syrian rebels, were killed in a bomb blast at the Atmeh border crossing with Turkey on Thursday, monitors and witnesses said.

At least 20 people were also wounded.

There were mixed reports of what caused the blast at the crossing in Idlib province, a rebel bastion west of Aleppo.

The Syrian Observatory for Human Rights said the blast was caused by an explosive belt, while the Amaq news agency, which is affiliated with "Islamic State," said the terror outfit carried out the attack with a car bomb.

Graphic images posted online from the scene showed the bloodied bodies of rebel fighters on the ground.

Among the dead was Sheikh Khaled al-Sayyed, the head of the top judicial body in rebel-held eastern Aleppo.

The rebels were from the Failaq al-Sham group, which is participating in Turkey's Operation Euphrates Shield further to the northeast along another section of the Turkish border.

Turkey launched Operation Euphrates Shield on August 24 to clear IS from a section of the border and block the US-backed Syrian Kurdish YPG militia from advancing further west. 

The intervention involves tanks, soldiers and artillery backed by US-led coalition airstrikes and rebel ground forces, which have been shipped from Idlib overland through Turkey to the frontlines along the border.

The intervention has succeeded in largely clearing IS from the border as Turkish-backed forces advance on jihadist-controlled Dabiq and al-Bab, two strategic towns.

Dabiq is a town where according to IS ideology an apocalyptic battle will take place. It is also the name of the terror group's English-language propaganda magazine.

cw/kl (AFP, AP, Reuters)