Militants from the Islamic State group have beheaded four tribesmen in eastern Syria, after accusing them of collaborating with the government, a monitoring group said Monday.

The men were Shaitat tribal members, a Sunni tribe which rose against IS in the summer, refusing to bow to militant control, after a deal between them collapsed, said the Syrian Observatory for Human Rights.

At the time more than 700 members of the tribe, most of them civilians, were killed by the jihadists, according to the Observatory, which relies on sources on the ground for its reports.

"On Sunday, IS beheaded four Shaitat tribesmen in the town of Albu Kamal... after accusing them in front of a crowd gathered to witness the execution, of collaborating with the regime," Observatory head Rami Abdel Rahman told AFP.

Albu Kamal lies on the border with Iraq in the IS-controlled oil-rich province of Deir Ezzor.

The Shaitat tribe, one of Syria's largest, rebelled against the IS group after it declared an Islamic caliphate in swathes of Syria and neighbouring Iraq in the summer.

IS emerged from Al-Qaeda's branch in Iraq and initially fought alongside Syrian rebel forces to bring down the government of Syrian President Bashar al-Assad.

But its brutality against civilians and rival fighters, including public beheadings of anyone who opposes its rule, has led to clashes with other anti-Assad groups.