Fighting between al Qaeda'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.
Fighters from the al Qaeda branch, the Nusra Front, also arrested a rebel commander from a more moderate group and several other insurgent leaders in a southern province, the Syrian Observatory for Human Rights said.
Infighting among insurgents has undermined the three-year-old rebellion against President Bashar al-Assad 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 Observatory said 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 Qaeda 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.
Al Qaeda's global leader Ayman al-Zawhri has called ISIL's entry into Syria's civil war a "political disaster" for Islamist militants there and urged the group to redouble its efforts in Iraq instead.
In the southern Deraa province, the Nusra Front arrested rebel commander Ahmed al-Neamah late on Saturday, accusing him of delivering the town of Kherbat Ghazalah to government forces, the Observatory said.
Neamah would be transferred, along with other detained commanders, to a local sharia, or Islamic law, court, the monitoring group said.
It said Neamah had been working to unify local rebel units and had appeared in an online video criticizing "extremists" and saying the Western-backed Free Syrian Army would rule Syria and ensure democracy.
Reporting by Alexander Dziadosz; editing by Andrew Roche/Ruth Pitchford