A rebel fighter stands in a building overlooking the damage from fighting in the city of Aleppo on Dec. 16, 2013.

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.

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