BEIRUT: Syrian rebels and Al-Qaeda launched a counter-offensive Saturday to expel the Islamist State of Iraq and the Greater Syria (ISIS) from Albu Kamal town on the Iraq border, a monitor said.

The operation came just days after some fighters from Al-Qaeda's Syria franchise, Nusra Front, pledged loyalty to ISIS in Albu Kamal, after it led an offensive in Iraq and seized chunks of territory

But not all 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 Nusra Front and Islamist rebels on one side and ISIS on the other," said the Syrian Observatory for Human Rights.

Nusra Front and its allies captured Saturday two ISIS positions in Albu Kamal, a key town in the oil-rich eastern province of Deir Al-Zor, said the Britain-based Observatory.

Nusra and other rebel groups in Syria have been locked in fierce fighting with ISIS since January that has killed thousands of fighters.

ISIS aims at setting an Islamic state that straddles Syria and Iraq and has taken control of a large part of Deir Al-Zor in recent weeks.

Rebels blame the West and other opposition backers for failing to provide them with more support to fight ISIS, which has captured swathes of territory north and west of Baghdad and seized weapons from fleeing Iraqi troops.

Syria's conflict began as a peaceful movement demanding political change, but became an all-out war after President Bashar Assad's regime unleashed a brutal crackdown against dissent.

A year into the fighting, foreign jihadists began to pour into Syria.

While ISIS 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.