The Opposition Observatory said at least eight opposition militants were killed overnight when jihadists from the Islamic State of Iraq and the Levant detonated a car bomb in Idlib governorate , adding that the car bomb targeted a rebel checkpoint near Ram Hamdan, northeast of Idlib city.
"Eight fighters from Islamist and other rebel brigades died after a huge bombing carried out by ISIL fighters," the Observatory said.
Clashes between coalitions of Islamist and opposition militants and their erstwhile allies ISIL erupted nearly two weeks ago, after a spate of accusations of abuses by the jihadist group.
Among those battling ISIL is the Islamic Front, a coalition of Islamist battalions, which issued the group an ultimatum late Monday night.
The statement said "we are setting ISIS a deadline of no more than 24 hours to stop its abuses, free the prisoners and hand over its weapons . If the group failed to heed the warning, it would deal with ISIL's presence and actions the same way as with the al-Assad regime."
Opposition militant groups have set ISIL similar deadlines in areas where the fighting in taking place, which the jihadist group has ignored.
The clashes were continuing on Tuesday, the Observatory said, including in Raqqa, a stronghold of ISIL.
Elsewhere, the Observatory said 18 people had been killed in the Ghouta neighborhood of Homs city in central Syria by shelling.