ISIL, a Sunni Islamist splinter of al Qaeda, has battled rival rebel groups in Syria for months and clashed occasionally with President Bashar al-Assad's forces.
Syria's civil war started with pro-democracy street rallies in 2011 but turned into an armed insurgency after a military crackdown on civilian protesters. Hardline factions like ISIL have gathered strength in the conflict, which has killed 160,000 people and displaced millions of people.