More than 230 bodies of people believed to have been killed by Islamic State of Iraq and Syria militants have been found in a mass grave in Syria's eastern Deir al-Zor province, a group monitoring the country's war said on Wednesday.
The bodies were believed to be members of the al-Sheitaat tribe which had battled ISIS militants, the Britain-based Syrian Observatory for Human Rights said.
Their deaths would bring the number of Sheitaat members killed by the ultra-hardline group to over 900.
ISIS 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 although 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 Observatory, which has tracked violence on all sides of the nearly four-year-old conflict, said beheadings were used to kill many of the tribe's members.
The tribe is from Deir al-Zor province and numbers about 70,000.
ISIS 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.