Fighting between an al-Qaida-linked group and a loose alliance of other rebel brigades has killed nearly 500 people over the past week in northern Syria, rebel supporters said.

The rebel-on-rebel violence broke out a week ago and has spread across four provinces in opposition-held parts of the north in what amounts to the most serious infighting among opponents of President Bashar al-Assad since the conflict began in March 2011.

The clashes pit fighters from a variety of Islamist and secular factions against the Islamic State of Iraq and the Levant group (Isil).

Isil and another al-Qaida-linked group, Jabhat al-Nusra, or the Nusra Front, initially joined forces with rebels fighting to oust President Assad's government.

However, the extremist Islamist group, although at first proving to be well-organised and efficient fighters and giving the ragtag rebels a military boost, eventually alienated many other rebels by using vicious tactics to implement its strict interpretation of Islamic law.

Britain-based Syrian Observatory for Human Rights admitted that at least 482 people had been killed in the fighting.

It said that 240 of the dead were rebel fighters, while another 157 were from Isil.

The remaining 85 were civilians, the Observatory said.

The consortium of rebel brigades has made headway against Isil fighters in several areas of the provinces of Aleppo, Idlib, Hama and Raqqa - although the al-Qaida-linked group has managed to regroup and curb some of its losses.