The operation came just days after some fighters from Al-Qaeda's Syria franchise, Al-Nusra Front, pledged loyalty to ISIL in Albu Kamal, after it led an offensive in Iraq and seized chunks of territory
But not all Al-Nusra fighters defected and those who refused to submit to the jihadist group joined forces with other Syrian rebel groups to launch the counter-offensive.
"Fighting has raged since late last night in Albu Kamal between Al-Nusra Front and Islamist rebels on one side and ISIL on the other," said the Syrian Observatory for Human Rights.
Al-Nusra and other rebel groups in Syria have been locked in fierce fighting with ISIL since January that has killed thousands of fighters.
ISIL aims at setting an Islamic state that straddles Syria and Iraq and has taken control of a large part of Deir Ezzor in recent weeks.
Rebels blame the West and other opposition backers for failing to provide them with more support to fight ISIL, which has captured swathes of territory north and west of Baghdad and seized weapons from fleeing Iraqi troops.
A year into the fighting, foreign jihadists began to pour into Syria.
While ISIL was initially welcomed by some rebels seeking Assad's ouster, its systematic abuses and quest for hegemony have turned the opposition, including Islamists, against it.
