A group linked to al-Qaeda recaptured much of its stronghold in the northeast Syrian city of Raqqa on Sunday, activists said, dealing a blow to rival opposition militants backed by Gulf Arab and Western states.
Fighters from the Islamic State of Iraq and the Levant (ISIL) battled remnants of rival Islamist units including the al-Nusra Front, another al Qaeda affiliate, in several neighbourhoods of Raqqa.
To the north, ISIL recaptured the town of Tel Abyad on the border with Turkey over the weekend.
Abu Khaled al-Walid, an activist speaking from the border area, said many fighters from Ahrar al-Sham, one of the Islamist groups, chose not to confront ISIL because the combatants were local people with little enmity for each other.
ISIL pulled out of Raqqa and other towns in northern Syria this month after an Islamist militants alliance attacked its strongholds, taking advantage of growing popular resentment of the group's foreign commanders and their drive to impose their strict interpretation of Islamic law.
But ISIL has regrouped in the last few days, using snipers, truck-mounted commando units and suicide bombers.
Opposition sources said the expertise of its foreign commanders, including a senior figure known as Omar al-Shishani, has been crucial to its advance.
In the province of Aleppo west of Raqqa, activists said ISIL had regained several rural towns, including Hrietan and Basraton, where ISIL killed a senior commander in the Nour al-Din Zanki brigades, a key unit in the newly-formed Mujahideen Army, which has been fighting ISIL in Aleppo.
Fighting also raged on Sunday between Western-backed al-Hr around the town of Renayan near Aleppo and in Urum to the east.