"We, the Army of the Mujahideen, pledge to defend ourselves and our honour, wealth and lands, and to fight ISIL, which has violated the rule of God, until it announces its dissolution," said the new alliance of eight groups, in a statement published on Facebook yesterday.
The alliance demanded that ISIL fighters either join the ranks of other rebel groups "or hand over their weapons and leave Syria."
The alliance accused ISIL of "spreading strife and insecurity... In liberated (rebel) areas, spilling the blood of fighters and wrongly accusing them of heresy, and expelling them and their families from areas they have paid heavily to free" from Assad's regime.
The newly formed group is made up of eight small to medium-sized brigades, and it was not immediately clear how many fighters it commanded.
But as the statement was issued it fought fierce clashes against ISIL in the Aleppo and Idlib provinces of northern Syria.
The Islamic Front, the largest rebel alliance, which is made up of several powerful Islamist groups, and the Syrian Revolutionaries Front, another major rebel bloc, also battled ISIL yesterday.
Assad's regime has long referred to all of its opponents -- peaceful activists and rebels alike -- as "terrorists" in a bid to deter more forceful Western intervention in the conflict.
