BEIRUT - Fighting between al Qaida's Syria branch and a splinter group in eastern Syria has forced more than 60,000 people to flee their homes, emptied villages and killed scores of fighters, a monitoring group said.

Infighting among rebels opposed to President Bashar al-Assad has undermined the three-year-old uprising against his rule and killed thousands of people since the start of the year.

The conflict has pitted hardline Islamists against more moderate insurgents, but disputes over turf and resources have also turned radical factions against one another, most recently in the oil-producing eastern province of Deir al-Zor.

The British-based Syrian Observatory for Human Rights said late on Saturday that al Qaida's Syrian wing, the Nusra Front, had taken over control of the town of Abreeha from the Islamic State of Iraq and the Levant (ISIL), a former al Qaida affiliate formally disowned by the group this year.

At least 62 fighters had been killed in around four days of clashes in the area, which have emptied Abreeha and the towns of al-Busayrah and al-Zir, whose populations total over 60,000, the Observatory said.

The Observatory, an anti-Assad group which monitors violence in Syria through a network of sources, said Islamist fighters had burned houses and a young girl had been killed by mortar fire during the fighting.

ISIL and the Nusra Front have clashed repeatedly over oilfields and strategic positions in Deir al-Zor, a desert province bordering Iraq. ISIL is a rebranding of al Qaida in Iraq and has fallen out with the global organisation over its role in Syria.

Al Qaida's global leader Ayman al-Zawhri has said ISIL's entry into Syria's civil war caused a "political disaster" for Islamist militants there and urged the group to redouble its efforts in Iraq instead.