The al-Qaeda-linked Islamist State of Iraq and the Levant executed dozens of rival Islamists over the last two days as the group recaptured most territory it had lost in the northeastern Syrian province of Raqqa, activists said.
One activist, speaking anonymously, said up to 100 fighters from the Nusra Front, another al-Qaeda affiliate, and the Ahrar al-Sham brigade were shot dead.
They had been captured by ISIL in the town of Tel Abyad on the border with Turkey, the nearby area of Qantari and the provincial capital city of Raqqa, he said.
"About 70 bodies, most shot in the head, were collected and sent to the Raqqa National hospital," the activist said. "Many of those executed had been wounded in the fighting. The fact that Nusra Front and Ahrar al-Sham are ideologically similar to the ISIL did not matter,"
There was no independent confirmation of the report.
Fighting between the ISIL and rival Islamists and more moderate rebels has killed hundreds of people over the last 10 days and shaken the hardline militant group led by foreign jihadists.
But the ISIL regrouped and recaptured much of its stronghold in Raqqa city on Sunday, activists said, dealing a blow to rival rebel groups backed by Gulf Arab and Western states.
Among those reportedly executed at the weekend was Abu Saad al-Hadram, Nusra Front's commander for Raqqa province, who was captured several months ago as tension mounted between the foreign-led ISIL and the more home-grown Nusra, opposition sources said.
In Raqqa, the only provincial capital under rebel control, activists said ISIL fighters battled remnants of rival Islamist units including the Nusra Front in several neighbourhoods.
To the north, ISIL recaptured Tel Abyad over the weekend. As a result, Turkish authorities closed a border crossing near the town and pulled out the facility's staff, according to the Syrian Revolution Co-ordinating Union, an opposition monitoring group.
Abdallah Farraj, a member of the opposition Syrian National Coalition from Raqqa, said rebels had been able to expel ISIL from parts of the neighbouring Aleppo province, but it would be hard to shake ISIL's hold on Raqqa and rural areas along key supply lines across the north.
"The rebels lack the organisation and the firepower to win. It will be difficult to defeat ISIL without military strikes from someone like Turkey," he said.
Abu Khaled al-Walid, an activist speaking from the border area, said many fighters from Ahrar al-Sham, one of the most powerful Islamist groups, chose not to confront ISIL because the combatants were local people with little enmity for each other.
"Many did not see a point in fighting their own relatives. ISIL is now in control of 95 per cent of Raqqa and its rural environs. Tel Abyad is also back with it," he said.
Raqqa, 385 kilometres northeast of Damascus, is the most significant city to have fallen completely to President Bashar al-Assad's opponents since the revolt against his family's four-decade rule broke out in March 2011.
ISIL pulled out of Raqqa and other towns in northern Syria this month after an Islamist rebel alliance attacked its strongholds, taking advantage of growing popular resentment of the group's foreign commanders, their killing of other rebels and a drive to impose a strict interpretation of Islamic law.
But ISIL has regrouped in the last few days, using snipers, truck-mounted commando units and suicide bombers.