Syrian activists say rebel infighting that swept opposition-held areas of the country's north has spread a city in the east, a key stronghold of an al Qaeda-linked group.

The recent rebel-on-rebel fighting in Syria is the strongest pushback yet by moderate and ultra-conservative rebels fighting to topple President Bashar Assad against radical extremist insurgents linked to al Qaeda.

The British-based Syrian Observatory for Human Rights says the rebels are clashing on Monday, Jan 6, with fighters of the Islamic State in Iraq and the Levant in eastern city of Raqqa, which has long been an ISIL stronghold.

The infighting erupted in the northern provinces of Aleppo and Idlib on Friday and has been spreading since. The push to the east suggests the rebels are preparing to overrun the ISIL completely.