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A car bomb likely planted by al Qaeda splinter group the Islamic State in Iraq and the Levant (ISIL) killed two rival rebel leaders and five others in eastern Syria, a group that monitors the violence said.
ISIL, a hardline Islamist group which has seized swathes of territory in Syria and Iraq, has been fighting rival insurgents for months to consolidate its grip on Syria's oil-producing Deir al-Zor province on the border with Iraq.
The infighting has killed more than 600 fighters and driven tens of thousands from their homes.
A car bomb in the town of al-Shamatiyeh killed seven people including a judge from the al Qaeda-linked Nusra Front and a commander of Ahrar al-Sham, another hardline Islamist group, late on Monday (local time), the Britain-based Syrian Observatory for Human Rights said.
Capturing parts of Deir al-Zor would help ISIL link up its territorial gains across Syria and Iraq, where it overran the country's second-biggest city of Mosul last week.
ISIL fought with the rival groups as it tried to advance towards the town of Busayra in the east of Deir al-Zor province on Monday, the Observatory said.
An ISIL fighter also detonated a suicide vest in an assassination attempt on the leader of another rebel brigade in al-Howayij in Deir al-Zor late on Monday, the Observatory said, saying the leader was wounded and several of his relatives killed.
Rebel infighting and the rise of hardline combatants has deterred Western states from providing arms to more moderate opposition fighters and undermined the three-year-old uprising against Assad which started as a protest movement.
The Observatory said last week that the Syrian branch of ISIL had put fighting on hold in Syria while it brought in weapons seized inside neighbouring Iraq.
Its fighters appeared to have held back in Syria last week, especially in their eastern stronghold near the Iraqi border, while their Iraqi wing was making rapid military gains.