The Islamic State militant group has executed 700 members of a tribe it has been battling in eastern Syria during the past two weeks, the majority of them civilians, a human rights monitoring group and activists said.

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

The conflict between Islamic State 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 Abdelrahman said. “Some were arrested, judged and killed.”

Reuters cannot independently verify reports from Syria due to security conditions and reporting restrictions.

Proclaiming a ‘caliphate’ straddling parts of Iraq and Syria, Islamic State 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, prompting the first US air strikes in Iraq since the withdrawal of American troops in 2011.

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

An activist in Deir al-Zor 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 Islamic State stormed the town earlier this week.

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