The jihadist Islamic State of Iraq and the Levant withdrew from the eastern Syrian province of Deir Ezzor on Monday after a three-day battle with Islamist rebels, a monitor said.
The Syrian Observatory for Human Rights said ISIL withdrew from the province, which borders Iraq, after fighting a coalition of opposition brigades including Al-Qaeda's Syria affiliate Al-Nusra Front.
ISIL is facing a widespread backlash from an array of rebel brigades angered by its abuses against civilians and rival opposition fighters.
Though it grew from Al-Qaeda's onetime Iraqi affiliate, Al-Qaeda chief Ayman al-Zawahiri has distanced his organisation from ISIL and ordered it to return to Iraq.
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ISIL's chief has ignored the admonition and continued to engage in clashes with rebels across opposition-held areas of Syria.
Al-Nusra had largely stayed out of the clashes with ISIL, but the Observatory said it joined around 10 other brigades in the fight against the jihadist group in Deir Ezzor.
The loss of Deir Ezzor province could be a serious setback for ISIL, as it holds oil reserves and is a key conduit for the jihadists to receive weapons and fighters from neighbouring Iraq.
More than 1,700 people have been killed in the clashes between rebels and ISIL that began in early January, according to the Observatory.