Eighteen fighters were killed in a car bomb blast that hit a Syrian insurgent group in the southern province of Quneitra on Wednesday, a monitoring group reported, and a rebel source said the attack was likely carried out by hardline Islamists.
The Syrian Observatory for Human Rights said the explosion took place in the village of al-Isha, hitting a base belonging to Jabhat Thuwwar Souria, a Free Syrian Army group.
Suhaib al-Ruhail, a spokesman for the Alwiyat al-Furqan group which operates in the area, said it was most likely carried out by "Daesh sleeper cells", a reference to Islamic State.
The incident did not appear to be related to the current cessation of hostilities between the Syrian government and its allies and non-jihadist insurgent groups.