An explosion in a village in northwestern Syria near the border with Turkey killed at least 20 people on Thursday, including several Turkey-backed opposition fighters, Syrian activists said.
The attack was quickly claimed by the Islamic State group.
The deadly bombing underscored the complex layers of the Syrian conflict, where a civil war between Syrian President Bashar Assad's forces and the rebels trying to oust him is taking place alongside a militant insurgency and an international war against the Islamic State group.
The Britain-based Syrian Observatory for Human Rights said the blast in the village of Atmeh was caused by a suicide bomber wearing an explosive belt. The activist group, which tracks the conflict through a network of activists on the ground, said tat along with 20 killed, many others were wounded in the explosion.
Another group, the Local Coordination Committees, said the blast was caused by a bag filled with explosives that went off on the Syrian side of a border crossing, killing 35 people including two senior judicial officials from the opposition.
The Observatory said the dead were Turkish-backed opposition fighters who have been fighting along with Turkish groups against the Islamic State group since August.
An amateur video posted online shows about 18 dead men lying on the floor outside what appears to be a clinic. Some of the men are in military uniforms. The video appears genuine and corresponds to other AP reporting of events depicted.
In the Islamic State group's claim, the IS-linked Aamaq news agency said a suicide car bomber struck a convoy of rebels on the Syrian side of a border crossing point in Atmeh.
Turkish Deputy Prime Minister Numan Kurtulmus described the blast as an "inhuman attack," adding that "those who perpetrated this committed a crime against humanity."
Turkish state-run Anadolu Agency said the attack occurred during a "change of guard" by the Turkish-backed forces at a refugee settlement.Turkey sent tanks into Syria on Aug. 24 as part of the so-called Euphrates Shield operation aimed at ousting Islamic State fighters and halting an advance by Syrian Kurdish forces, which Ankara sees as allies of Turkey's own outlawed Kurdish militia that has been waging a 30-year insurgency inside Turkey.