The victims were from the al-Sheitat tribe whose land was overrun last month by the jihadists.

Militants of the Islamic State, IS, formerly known as the Islamic State of Iraq and the Levant, ISIS, have over the past two weeks executed 700 members of a Syrian tribe, the majority of them civilians, a human rights monitoring group said over the weekend.

Al Jazeera TV cited the Syrian Observatory for Human Rights as saying beheadings were used to execute many of the al-Sheitaat tribes people in Deir al-Zor Province. The conflict between IS and the al-Sheitaat tribe, who number about 70,000, flared up after the armed group took over two oil fields last July.

An activist in Deir al-Zor told news agencies that 300 men were executed in one day in the town of Ghraneij, one of the three main towns of the al-Sheitaat tribal heartland, when the militants stormed the town earlier last week. Another source said residents of al-Sheitaat towns were given three days to leave.

Sheik Rafaa Aakla al-Raju, leader of the al-Sheitaat tribe, appealed in a video message released last week to 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,” he warned. Proclaiming a 'caliphate' straddling parts of Iraq and Syria, IS has swept across northern Iraq in recent weeks, pushing back Kurdish regional forces and driving tens of thousands of Moslems, Christians and members of the Yazidi religious minority from their homes.

This has prompted the first US air strikes in Iraq since the withdrawal of American troops in 2011. The insurgents are also tightening their grip in Syria, of which they now control roughly a third, mostly rural areas in the north and east.