Over the past two weeks, armed group the Islamic State (IS) has executed 700 members of a Syrian tribe, the majority of them civilians, a human rights monitoring group said on Saturday.

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

A conflict between IS and the al-Sheitaat tribe, who number about 70,000, flared after the armed group took over two oil fields in July.

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

News agencies could not independently verify reports from Syria due to security conditions and reporting restrictions. Abdelrahman, meanwhile, said that the fate of 1,800 other members of the tribe was unknown.

Sheik Rafaa Aakla al-Raju, leader of the al-Sheitaat tribe, called in a video message issued last week for other tribes to join the fight against IS.

“We appeal to the other tribes to stand by us because it will be their turn next...If (Islamic State) are done with us the other tribes will be targeted after al-Sheitaat. They are the next target,” he said.

IS has captured most of Deir al-Zor and declared it to be part of its "caliphate," along with large swathes of territory it has captured across the border in Iraq.

SOHR said the al-Sheitaat tribe had previously agreed not to oppose the armed group's authority, in exchange for it not harassing or attacking its members.

Fighting between IS and the tribe erupted after the deal between the two sides collapsed, with the tribe refusing to bow to Islamic State’s control.

Wire services