BEIRUT, Dec 17 (Reuters) - More than 230 bodies of people believed killed by Islamic State militants have been found in a mass grave in Syria's eastern Deir al-Zor province, activists said on Wednesday.
The bodies were thought to be members of the al-Sheitaat tribe which had battled Islamic State militants, the British-based Syrian Observatory for Human Rights and activists monitoring the conflict said.
Their deaths would bring the number of Sheitaat members said to have been killed by the ultra-hardline Islamist group to over 900.
Omar Abu Layla, a spokesman for the moderate rebel umbrella Free Syrian Army group in Syria's east, said Sheitaat tribespeople had discovered the mass grave as they returned to their homes. Islamic State, occupying the area, had given them permission to return.
"This is a message from Daesh that if there is any attempt at revenge, your fate will be the same as your relatives," he said, using a derogatory Arabic acronym for the Islamic State group.
Pictures shared by Abu Layla and activists on social media said to be of the mass grave showed decaying bodies and bones covered in dust and dirt lying amid rocks and sand.
Islamic State militants control all but a few pockets of Deir al-Zor province, which borders territory also under its control in Iraq.
The province's oilfields have been a major source of revenue for the group, but its operations have been under pressure since a U.S.-led coalition started launching air strikes against it in Syria in September.
In August, activists said the militant group had killed some 700 members of the Sheitaat tribe, the majority of them civilians, over the preceding two weeks after conflict flared when the militants took over two oilfields.
The people in the newly-discovered mass grave are believed to have been killed around the same time, activists said.
The Observatory, which has tracked violence on all sides of the nearly four-year-old conflict, said many of the Sheitaat victims had been beheaded.
The tribe is from Deir al-Zor province and numbers about 70,000.
Islamic State fighters are currently battling Syrian President Bashar al-Assad's forces near Deir al-Zor city for a military air base that is one of the government's last strongholds in the country's east. (Reporting by Alexander Dziadosz; Editing by Dominic Evans)