Members of the Shuayat tribe battling
The corpses – many decapitated and otherwise mutilated - were discovered by members of the tribe allowed to return to the village of Kashkashiyeh on Friday after registering with Islamic State as part of a deal brokered with the extremists, according to the Syrian Observatory for Human Rights, a Britain-based activist network.
Islamic State's leader, Abu Bakr Baghdadi, also stipulated that the Sunni Muslim tribesmen could not assemble, must take religious lessons, adhere to a curfew and inform on all "apostates," or traitors who would then probably face death, according to a United Nations report.
The mass killing of the tribesmen, which was reported Wednesday, was viewed as punishment for fighting the Sunni extremist group in the eastern province of Dair Alzour, where it has been gaining ground.
Activists posted photographs online this week of graves that showed dusty skulls and corpses partially buried in dirt ditches.
The mass graves were also reported by the official Syrian Arab News Agency, which denounced Islamic State as having committed "crimes against humanity."
The discovery of the bodies, which could not be independently verified, would raise the tribe's death toll in recent months to about 900.
Islamic State has proved ruthless against fellow Sunnis belonging to tribes that resist. Last month the militants reportedly executed hundreds of Albu Nimr tribesmen in Iraq's western Anbar province, lining them up and shooting them in the head.
In Dair Alzour, Islamic State routed government and rebel forces after a stunning advance during the summer that netted large swaths of Syria and Iraq. In July, they mounted a bid for oil wells in an area 15 miles southeast of Dair Alzour that culminated the following month in a massacre of 700 Shuayat tribesmen, the United Nations reported.
In late summer, a U.S.-led coalition began targeting the group with airstrikes in Iraq and then Syria. The attacks continued Thursday, with six in Syria and five in Iraq, U.S. military officials said, destroying an Islamic State building, tactical units, vehicles and fighting positions.