Among the members of the Shaitat tribe killed were 100 fighters, but the rest were civilians, the Syrian Observatory for Human Rights said on Saturday.
They were killed in the Ghranij, Abu Hamam and Kashkiyeh villages of the mainly IS-controlled province of Deir Ezzor, said the Observatory, which relies on a vast network of activists and medics on the ground for its information.
Head of the monitoring group Rami Abdel Rahman said the fate of 1800 other members of the tribe was unknown.
Fighting between the Islamic State group members and the Sunni Muslim tribe erupted in the oil-rich province after a deal between the two sides collapsed, with the tribe refusing to bow to IS control.
The clashes broke out after IS detained three members of the tribe, "violating an agreement", the monitor said.
The IS has captured most of Deir Ezzor and declared it to be part of its "caliphate," along with large swathes of territory it has captured across the border in Iraq.
The Observatory said the Shaitat had vowed not to oppose the Islamic State's authority, in exchange for the IS not harassing or attacking its members.
On August 5, the Observatory said IS beheaded three Shaitat tribesmen and put their heads on public display.
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