(Adds rebels blowing up ISIL base, weekend death toll)
By Erika Solomon
BEIRUT, Feb 10 (Reuters) - An al Qaeda splinter group has withdrawn its forces from Syria's oil-rich eastern province of Deir al-Zor, activists and rebels said on Monday, after days of heavy fighting with its rivals.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 Deir al-Zor.
"The ISIL fighters have almost completely withdrawn from Deir al-Zor.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 an ISIL stronghold.
ISIL activists on Twitter said the group had pulled out of Deir al-Zor to prevent further bloodshed among rebel factions who are supposed to be fighting Syrian President Bashar al-Assad's forces.
ISIL's supporters said the estimated 200 fighters leaving Deir al-Zor would probably turn to assassinations and car bombings against the remaining rebel groups in the province - a tactic the group has used in other opposition-held areas.
Several Islamist and more secular rebel groups teamed up last month for an offensive to try to push their former ISIL allies out of rebel-held regions in northern and eastern Syria.
Activists in Deir al-Zor posted videos on the Internet that showed the main ISIL headquarters in the province collapse into a cloud of dust as rebels blew up the building.
ISIL, which has attracted many foreign militants into its ranks, is a small but powerful fighting force in Syria, and also operates in neighbouring Iraq. It has alienated many civilians and opposition activists by imposing harsh rulings against dissent, even beheading its opponents, in areas it controls.
More than 2,300 rebels have been killed in the past month of infighting, making it the bloodiest such episode since the Syrian conflict began nearly three years ago.
Peaceful anti-Assad protests in March 2011 drew a violent response from the security forces, leading to an armed revolt that degenerated into a civil war now estimated to have killed more than 130,000 people and forced millions to flee their homes.
Dozens of people were killed in fighting between ISIL and its rivals at the weekend. In one incident, an ISIL suicide car bomber blew himself up among a crowd of civilians and fighters near a market, according to the Syrian Observatory for Human Rights, killing more than 20, including six children.
ISIL is a rebranded version of al Qaeda's affiliate in Iraq, but it has defied the main network's requests to limit itself to fighting there instead of Syria. Al Qaeda's central leadership formally announced a split with ISIL earlier this month.
The Observatory, a British-based pro-opposition monitoring group, said Deir al-Zor was now in the hands of Nusra fighters as well as those from 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 captured by rebels on Monday.
Unlike other Islamist groups such as Nusra, which share similar austere interpretations of Islam, ISIL has tried to set up an Islamic caliphate in territory it has seized in Iraq and Syria. Other Syrian rebels want to topple Assad before deciding on a ruling system, though many also want an Islamic government. (Editing by Alistair Lyon)