The jihadist Islamic State of Iraq and the Levant signed a truce deal in Syria with an Islamist militant brigade involved in a widespread armed backlash against it.
The deal signed Tuesday between ISIL and Suqour al-Sham was posted online today, and also reported by the opposition observatory.
It calls for "an immediate halt to fighting between the sides and no assault by either side on the other in any way".
It also urges that any disagreements between the groups be referred to an Islamic court.
Jihadist groups were initially welcomed to the Syrian conflict by some in the opposition.
But ISIL has sparked a fierce backlash from both moderate and Islamist brigades because of alleged abuses against civilians and rival armed opposition groups.
Since January 3, a coalition of those militant groups has been fighting ISIL across areas under opposition control, including Idlib, Aleppo and Raqa governorate .
The fighting has killed more than 1,700 people, according to the observatory.
ISIL grew from the Islamic State of Iraq, a one-time Al-Qaeda affiliate in Iraq that expanded into Syria after the conflict there began.
The group sought to merge with the jihadist Al-Nusra Front in Syria, but the Front rebuffed the overture and pledged allegiance directly to Al-Qaeda chief Ayman al-Zawahiri.
Zawahiri says Al-Nusra is Al-Qaeda's official branch in Syria, and has distanced his organisation from ISIL, unsuccessfully ordering them to return to Iraq.
Al-Nusra has largely stayed out of the fighting against ISIL, but an array of moderate brigades as well as Islamist groups from the Islamic Front - including Suqour al-Sham - have been involved in fierce clashes with ISIL.