The Syrian Observatory for Human Rights, which has tracked violence on all sides of the three-year-old conflict, said on Saturday that reliable sources reported beheadings were used to execute many of the al-Sheitaat tribe, which is from Deir al-Zour province.

The conflict between ISIL and the al-Sheitaat tribe, who number about 70,000, flared after the militants took over two oil fields in July.

“Those who were executed are all al-Sheitaat,” Observatory director Rami Abdel-Rahman said by telephone from Britain. “Some were arrested, judged and killed.”

Straddling parts of Iraq and Syria, ISIL has swept across northern Iraq in recent weeks, pushing back Kurdish regional forces and driving tens of thousands of Muslims, Christians and members of the Yazidi religious minority from their homes.

The insurgents are also tightening their grip in parts of Syria, of which they now control roughly a third, mostly rural areas in the north and east.

An activist in Deir al-Zour who spoke on condition of anonymity told Reuters that 300 men were executed in one day in the town of Ghraneij, one of the three main towns of the al-Sheitaat tribal heartland, when ISIL stormed the town earlier this week.

Another opposition activist from Deir al-Zour said residents of al-Sheitaat towns had been given three days to leave. Civilians fleeing al-Sheitaat towns had either taken sanctuary in other villages or travelled to Iraq, he said.

More than 170,000 people have been reportedly killed in Syria's conflict, which pits overwhelmingly Takfiri militants against the Syrian government, since March 2011.

NTJ/HH