At least 16 Syrian militants were killed on Sunday in a double bomb attack carried out by the jihadist Islamic State of Iraq and the Levant, an NGO said.
The opposition observatory said another 20 militnats were injured in the attack, some of them seriously, noting that the attack took place in the town of al-Ra'ei, not far from the Syrian border with Turkey.
The Observatory said an ISIL fighter blew himself up at the headquarters of an Islamist militant brigade in the Aleppo town, near the border with Turkey.
A car bomb detonated outside at the same time, the group said.
The militants were Islamist brigades that have joined opposition militants 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 ISIL that has raged throughout militant-controlled areas.
The bomber had reportedly offered to negotiate with the militants in the name of ISIL, but detonated an explosive belt inside the headquarters.
The opposition militant-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 ISIL has found itself increasingly embattled, after facing accusations of abusing rival militants and civilians in militant-held areas and focusing on consolidating its rule rather than fighting Syrian Army.