ISIL terrorists have killed more than 700 members of a tribe in eastern Syria in two weeks, the Syrian opposing UK-based Observatory said on Saturday.

ISIL terrorists have killed more than 700 members of a tribe in eastern Syria in two weeks, the Syrian opposing UK-based Observatory said on Saturday.

Among the members of the Shaitat tribe killed were 100 fighters, but the rest were civilians, the monitoring group said. They were killed in several villages in the mainly ISIL-controlled eastern province of Deir Ezour.

They were killed in the Ghranij, Abu Hamam and Kashkiyeh villages of the mainly IS-controlled province of Deir Ezour, said the Observatory, which relies on a vast network of activists and medics on the ground for its information.

Observatory head Rami Abdel Rahman said that the fate of 1,800 other members of the tribe was unknown.

Fighting between the ISIL terrorists and the Sunni Muslim tribe erupted in the oil-rich province after a deal between the two sides collapsed, with the tribe refusing to bow to ISIL control.

The clashes broke out after ISIL detained three members of the tribe, "violating an agreement", the monitor said.

The ISIL has captured most of Deir Ezour and declared it to be part of its "caliphate," along with large swathes of territory it has captured across the border in Iraq.

The Observatory said the Shaitat had vowed not to oppose the ISIL's authority, in exchange for the ISIL not harassing or attacking its members.

On August 5, the Observatory said ISIL beheaded three Shaitat tribesmen and put their heads on public display.