(Adds details, background)
BEIRUT, Feb 10 (Reuters) - An al Qaeda splinter group has withdrawn its forces from Syria's oil-rich eastern province of Deir al-Zor, activists and rebels said on Monday, after days of heavy fighting with its rivals.
Rebel groups, including al Qaeda's Syrian affiliate the Nusra Front, have been battling the Islamic State of Iraq and the Levant (ISIL) for control of towns and oilfields in the area, sparking a spate of car bombs in the province.
"The ISIL fighters have almost completely withdrawn from Deir al-Zor. The fighters are moving to Hassaka and Raqqa (provinces)," said a source from the Nusra Front, who asked not to be named. Raqqa remains the stronghold of ISIL.
Pro-ISIL activists on Twitter said the group had withdrawn from Deir al-Zor to prevent further bloodshed.
Several rebel groups launched a campaign last month to try to push ISIL forces, their former allies, out of opposition-held regions in northern and eastern Syria.
Islamist opposition groups joined forces with some secular rebel units to fight ISIL, with whom they have territorial disputes and ideological differences.
ISIL, which has attracted many foreign Islamist militants into its ranks, is a small but powerful fighting force in Syria's opposition areas. It has alienated many civilians and opposition activists, however, by imposing harsh rulings against dissent in areas it controls, such as beheadings.
Over a month of clashes killed more than 2,300 rebels, making it the bloodiest episode of infighting in Syria's nearly three-year conflict.
Syria's crisis began as street protests against President Bashar al-Assad but grew into an armed insurgency after his security forces cracked down on demonstrations.
It has now degenerated into a civil war that has killed more than 100,000 people, forced millions to flee their homes, and is destabilising neighbouring countries as well.
ISIL is the rebranding of al Qaeda's affiliate in neighbouring Iraq, but it defied the central leadership's requests to limit itself to fighting there instead of Syria. Al Qaeda's central leadership formally announced a split with ISIL earlier in February.
The Syrian Observatory for Human Rights, a pro-opposition monitoring group, said Deir al-Zor was now in the hands of Nusra fighters as well as 10 other rebel groups.
"There were heavy clashes. ISIL asked for mediation but the Nusra Front rejected that, so it pulled out," he said.
Some activists said one of ISIL's Deir al-Zor leaders, known as Abu Ther al-Iraqi, was also detained by rebels on Monday.
Efforts to mediate between ISIL and other rebel groups, even Islamist forces with similar religious views, have failed.
Unlike other Islamist groups such as Nusra, which follow similar austere interpretations of Islam, ISIL was trying to set up an Islamic caliphate in territory it seized in Iraq and Syria. Other Syrian rebel forces want to topple Assad and then determine a ruling system for Syria, though many want an Islamic government. (editing by Elizabeth Piper)