The bodies of 230 people killed by the Islamic State in Iraq and the Levant (ISIL) group have been found in a mass grave uncovered by their relatives in Syria's Deir Ezzor province, a monitoring group has said.

The discovery brings the number of members of the Shaitat tribe killed during ISIL’s summer advance in Deir Ezzor, near Iraq, to more than 900, the Syrian Observatory for Human Rights said on Wednesday.

"The Syrian Observatory for Human Rights has learned from trusted sources that more than 230 bodies have been found in a mass grave in the desert near al-Kashkiyeh in the east of Deir Ezzor," said the Britain-based group.

It said the "vast majority" were civilians, and that many of them were executed in cold blood after the tribe rose up against ISIL. The tribe had earlier driven out rival self-declared jihadists and rebels from the area.

ISIL controls large swathes of northern and eastern Syria, as well as parts of neighbouring Iraq.

Hundreds more members of the Shaitat tribe are still missing, said the Observatory, which relies on a large network of activists, doctors and military sources on all sides of the Syrian conflict for its reports.

The tribespeople discovered the grave as they returned to their villages from months of displacement after losing their battle against ISIL.

They have been allowed to return only after agreeing to respect an ISIL-imposed curfew, as well as a prohibition on gatherings and weapons.

Anyone who fights ISIL, according to the group’s rules, will be considered a heretic and executed.

Source: Agencies