An Al-Qaeda front group fighting in Syria on Wednesday overran the northern town of Azaz near the border with Turkey after fierce clashes with rebels, activists told AFP.
"The Islamic State of Iraq and the Levant (ISIS) has seized complete control of Azaz. They are in control of the town's entrances," said Abu Ahmad, an activist inside the town.
Another activist, a resident of the border town, confirmed the report.
"ISIS are in full control. They are detaining people from their homes, and they are manning all the checkpoints," he told AFP via the Internet, speaking on condition of anonymity.
The Syrian Observatory for Human Rights was more cautious in its reporting on ISIS's advances in Azaz, and its Britain-based director Rami Abdel Rahman said the group was not yet in full control.
"ISIS snipers are deployed all over Azaz. They have made major advances," he told AFP.
Abdel Rahman also said "there is growing popular resentment against ISIS," amid multiplying reports of kidnappings, summary executions and other abuses in areas where they operate.
The development in Azaz comes amid increasing reports of fighting in northern Syria between ISIS and non-jihadist rebels vying to bring down President Bashar al-Assad's regime.
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In a sign that Syria's conflict has grown highly localised in recent months, ISIS has fought side by side with rebels against government troops in other areas of the country.
In Azaz, reports emerged of continuing clashes.
Abu Ahmad said that after seizing the town, ISIS fighters were "on their way to the (nearby) Bab al-Salameh" border crossing with Turkey.
Bab al-Salameh has for months been under control of local rebels, chiefly the Northern Storm brigade.
News of the reported seizure of Azaz came after a battle lasting several hours between Northern Storm fighters and ISIS.
Among those reported killed in the clashes was Omar Diab, a media activist from Azaz.
Better known by his pseudonym Hazem al-Azizi, Diab's killing triggered an outpouring of anger against ISIS on activists' online platforms.
The Northern Storm brigade meanwhile issued an online statement calling on other rebel groups nearby to "move directly towards Azaz, to protect the town and free it from ISIS."