Syrian rebels and al Qaeda-linked fighters battled near the border with Turkey on Wednesday, activists said, in an outbreak of violence that exposes serious divisions between factions fighting President Bashar al-Assad.
The al Qaeda-linked Islamic State of Iraq and the Levant (ISIL) took control of the border town of Azaz last month, kicking out rival rebels and prompting Turkey to shut its border crossing about 5 km (3 miles) away.
ISIL, which wants to merge Syria into a larger state ruled by Islamic law, has maintained control of the town since then and clashes have periodically erupted between them and fighters of the Northern Storm brigade they expelled to its outskirts.
The Britain-based Syrian Observatory for Human Rights, which has a network of sources across the country, said heavy clashes had resumed on Wednesday between the two groups.
It said there were reports ISIL fighters were advancing toward Northern Storm bases and checkpoints near the crossing into Turkey and in villages in the outskirts of Azaz.
An activist in the area confirmed fighting had started, but did not have details.
The Syrian rebels have been undermined by infighting, partially over conflicting ideology, but more often over territory, spoils of war and control of resources and smuggling.
ISIL, which is also present in neighbouring Iraq, comprises a larger number of foreign fighters than other hardline Islamist brigades fighting in Syria.
More than 100,000 people have died in Syria's conflict, which started as a peaceful protest movement against four decades of Assad family rule but turned into a war after a government crackdown.