BEIRUT (Reuters) - As Islamic State militants advanced in the Syrian province of Deir al-Zor last month, those who had held out against them faced a simple choice: beg for mercy or face certain death.

Their options were laid out in stark religious terms by the militant Islamists who are trying to carve out their own state in Syria and Iraq. Defeated fighters were required to "atone" or die, a choice set out in Islamic terms and implying that resisting Islamic State rule amounted to a sin against God.

"I surrendered my weapons," said a rebel fighter who capitulated to Islamic State on July 2 and has been living in fear for his life ever since. He still believes Islamic State could execute him at any moment. "Everyone is subject to this. Everyone is afraid," he said, speaking via internet link.

In cementing its control over the oil-producing province of Deir al-Zor, Islamic State has unleashed one of its bloodiest waves of repression to date, employing mass executions, threats and house demolitions as the attention of Western states has focused on rolling back the group in neighboring Iraq.

While some have been granted a pardon on Islamic State's terms that require complete allegiance, others have been shown no mercy. One tribe in particular has been singled out for persecution.

Hundreds of members of the Sheitaat clan have been executed after their tribe refused to submit to Islamic State. The entire tribe have been deemed "hostile apostates" by the group, an offshoot of al Qaeda that has declared a "caliphate" in the territory it holds.

Their killings are a reminder, say locals, that many of Islamic State's victims are not minority Shi'ites, Yazidis or Christians, but Sunnis who - nominally at least - follow the same denomination of Islam.

As Islamic State's advances in Iraq have sent minorities fleeing for fear of execution or forced conversion to Islam, the group's radical interpretation of the religion has, in the case of the Sheitaat, laid grounds for mass persecution of Sunnis.


INFIDELS

Islamic State has declared the Sheitaat tribe "an unbelieving sect" that should be fought as if they were infidels, according to a report from the Syrian Observatory for Human Rights, which tracks violence in the Syrian war.

At least 700 hundred members of the tribe have already been executed, the Observatory reported on Aug. 16.

Another 1,800 are still missing after being detained by Islamic State, according to the Observatory, which gathers information from all sides in the Syrian war. Its efforts to pledge allegiance to Islamic State have been rebuffed.

Pictures of the bodies of men apparently slain by Islamic State fighters in Sheitaat areas are surfacing every day, said Rami Abdelrahman, founder of the Observatory. "We have repeatedly expressed concerns about extermination," he said.

"It is the first time that the Islamic State has used these (religious) concepts against an entire tribe," he said.

It has provided a convenient religious tool for crushing a tribe that until recently controlled several oil fields in Deir al-Zor, according to a source familiar with the conflict. That revenue stream is now fully in the hands of Islamic State.

The treatment of the Sheitaat has served as a powerful deterrent to further rebellion in Deir al-Zor, a province bordering Iraq in the east mostly inhabited by Sunni Arabs many of whom are members of tribes that span the border into Iraq.

Besides the Sheitaat tribe, thought to number about 150,000 people, the Islamic State has accepted the surrender of other influential clans in the area which have publicly capitulated to the group.

It was in early July that Islamic State, boosted by territorial gains in Iraq, staged a rapid advance in the province, securing a corridor of Syrian territory along the Euphrates river all the way to the Iraqi border.


RAMADAN