Damascus. At least 482 people have been killed in clashes between Syrian rebels and the al-Qaeda-linked Islamist State in Iraq and the Levant (ISIS), activists say.The Syrian Observatory for Human Rights said the toll comprised 85 civilians, 240 rebels and 157 ISIS fighters.ISIS had killed 42 prisoners in Aleppo, while 47 members of the jihadist group had been executed by rebels, it added.The fighting has spread across four provinces in rebel-held parts of northern Syria over the past week.Attacks on fellow rebels and the abuse of civilian opponents of President Bashar al-Assad’s government by ISIS’s predominantly foreign fighters have led to increasingly frequent confrontations in recent months.The latest clashes erupted last Friday when rebels led by the Islamic Front, a relatively new coalition of Islamist groups, launched what appeared to be a series of co-ordinated strikes against ISIS. The offensive was backed by the opposition National Coalition.Islamist brigades captured ISIS’s headquarters in the northern city of Aleppo on Wednesday. At the former children’s hospital they found the bodies of several men who had been executed.On Friday, rebels were reportedly making advances against ISIS in Aleppo and Idlib provinces, where ISIS’s presence was relatively weak, but struggling in the ISIS stronghold of Raqqa, large parts of which have been under the jihadists’ control for months.Rami Abdul Rahman, the director of the Syrian Observatory, told the AFP news agency: "It is likely dozens more people have lost their lives, but it is impossible to accurately document all the killings.”On Tuesday, an ISIS spokesman warned its rivals that it would "crush them completely and kill the conspiracy in its cradle”.