At least 16 Syrian rebels were killed on Sunday in a double bomb attack carried out by the State of Iraq and the Levant, an NGO said.
Another 20 rebels were injured in the attack, some of them seriously, the Syrian Observatory for Human Rights said.
The attack took place in the town of al-Ra'ei, not far from the Syrian border with Turkey, the group said.
The Observatory said an ISIS fighter blew himself up at the headquarters of an Islamist rebel brigade in the Aleppo town, near the border with Turkey.
A car bomb detonated outside at the same time, the group said.
The rebels were brigades that have joined moderate rebels in fighting the jihadist ISIL since the beginning of a backlash in early January.
The Observatory said the bombings came as fighters gathered at the headquarters to discuss the possibility of a truce in the fighting with ISIS that has raged throughout rebel-controlled areas.
The bomber had reportedly offered to negotiate with the rebels in the name of ISIL, but detonated an explosive belt inside the headquarters.
The rebel-jihadist clashes have killed around 1,400 people since they began, although the Observatory says the real toll is likely much higher because both sides are secretive about their losses.
Some in Syria's opposition initially welcomed foreign jihadists to the conflict, valuing their battlefield experience, weapons and funding.
But ISIS has found itself increasingly embattled, after facing accusations of abusing rival rebels and civilians in rebel-held areas and focusing on consolidating its rule rather than fighting the regime.
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