BEIRUT (AFP) - Rebels battling jihadists in northern Syria have killed or captured scores of the militants loyal to an Al-Qaeda affiliate in two days of fighting, a watchdog said on Saturday.

"At least 36 members and supporters of the Islamic State of Iraq and the Levant (ISIL) have been killed since Friday in Idlib (northwestern province) and more than 100 have been captured by rebels" in Idlib and Aleppo, the Syrian Observatory for Human Rights said.

The reports come a second day into major clashes in opposition areas of Aleppo and Idlib provinces between ISIL and three rebel alliances, including the massive Islamic Front.

Jihadists who flocked to Syria to join the rebels in their fight against President Bashar al-Assad's regime were welcomed at first.

But relations grew bitter after ISIL fought other opposition groups for control and committed systematic abuses against activists and rival rebels.