Beirut, Lebanon: A car bomb killed at least 18 members of a Western-backed rebel coalition in southern Syria on Wednesday, a monitoring group said.

Four leaders of the Syrian Revolutionaries Front were among the dead after unknown assailants targeted the group in a village in Quneitra province, the Syrian Observatory for Human Rights reported.

The rebel coalition, which is also backed by Arab countries including Jordan, is one of the most powerful in Quneitra province along with Al-Qaeda-affiliated Al-Nusra Front.

Together with other rebels, it drove Islamic State group jihadists from the province a year ago.

The United States announced in September 2014 it wanted to support and train members of the Syrian Revolutionaries Front.

The alliance was created in late 2013 after the establishment of the Islamic Front which brings together various strains of Islamists.

A fragile ceasefire in Syria has been in place since Saturday but it does not include jihadist groups.