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.

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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

"I will never forget that day," said the rebel, describing how Islamic State fighters seized the town of Shuhail, 40 km (25 miles) southeast of Deir al-Zor city, during the Islamic holy month of Ramadan, when Muslims fast from dawn to dusk.

"People were about to break their fast when Islamic State launched its attack," said the rebel, who has spoken to Reuters regularly over the past year or so.

He spoke to Reuters via online link from inside his car, which he had parked in front of an internet cafe to connect to the Wifi signal. The Islamic State had confiscated his personal modem.

"Islamic State is carrying out a detailed survey of all the men. They are recording full names, and marking the ones who fought them, like me, with detailed information about which weapons I used and which battles I fought.

"We don't know the purpose of this survey. Are they going to forcefully enlist us in their ranks? Are they going to keep an eye on us for their security? Are they going to seize our property like they’re already doing?" he asked.

The rebel, whose name was withheld for security reasons, was a fighter in a group linked to the Free Syrian Army, an umbrella organization for Western-backed rebels who have largely faded from the scene as better armed Islamist groups have prevailed.

The Nusra Front, al Qaeda's affiliate in the Syrian civil war, also took part in the effort to repel Islamic State's advance in Deir al-Zor. It has now abandoned an area that was once a stronghold, having refused to capitulate.

Sensing defeat was at hand, weaker rebel battalions called in a mediator to tell Islamic State they were ready for a truce and were even prepared to pledge allegiance to the group.

"We all knew that if this battle continued all night, it would be the end of us all - our town, our families," said the rebel, speaking of the night during Ramadan.

Islamic State spared rebel fighters and local clan elders who surrendered in Shuhail. But they also made the town's 35,000 residents leave for eight days with the stated aim of searching homes and securing the area.

"We had no choice. All of us, we left the city: elderly, babies, civilians, fighters, everyone," the rebel fighter said.

Some stayed with relatives in nearby villages, while others slept in the desert for a week, enduring the harsh summer heat.

"I've atoned, but I don't support them," added the rebel.

ABANDONED HOMES

But for the Sheitaat clan, there has been no mercy.

Three Sheitaat villages seized by Islamic State have been designated as a military zone, the rebel and another activist from the area said. The clan's property and livestock have also been seized, another person from the area said.

Islamic State has declared that no truce is possible with the Sheitaat, that its prisoners can be killed, and its women are unfit for marriage, according to the Observatory.

"We're still seeing Islamic State trucks loaded up with furniture and rugs from Sheitaat homes in those villages, which are now totally abandoned," said one person from the area contacted by internet link, speaking on condition of anonymity.

Islamic State has started to use house demolitions as a punishment. A video posted over the weekend shows what appears to be the detonation of a rural home as the narrator, who identifies himself as from Islamic State, explains that the home belongs to Sheitaat "apostates".

"This is a warning to all clans to capitulate and acquiesce into obedience of Islamic State," he says.

Other Deir al-Zor tribes have issued a plea to Islamic State to spare the Sheitaat. Clan and tribal elders from other groups appeared in a video posted on Sunday appealing to Islamic State leader Abu Bakr al-Baghdadi to pardon them.

"We ask that you extend forgiveness to the people of Sheitaat, who have pledged allegiance to Islamic State ... and your forgiveness will facilitate a new phase," said one of the leaders, dressed in traditional tribal robes.

Alluding to the possible expansion of U.S. air strikes on Islamic State into Syria, he says that "the Crusader enemy is preparing to hit the Sunni areas that fall under the rule of Islamic State".

"So let's be one hand together against those hostile to God’s religion."

(Reporting by Beirut bureau; Editing by Tom Perry/Sylvia Westall/Giles Elgood)