The Britain-based group said it had "learned from trusted sources that more than 230 bodies have been found in a mass grave in the desert near Al-Kashkiyeh in the east of Deir Ezzor".
The "vast majority" were civilians, many of them executed in cold blood after the tribe rose up against the Islamic State after it had driven out rival jihadists and rebels from the area.
Hundreds more members of the Shaitat tribe are still missing, said the Observatory, which relies on a large network of activists, doctors and military sources on all sides of the Syrian conflict for its reports.
The tribespeople discovered the grave as they returned to their villages from months of displacement after losing their battle against IS.
They have been allowed to return only after agreeing to respect an IS-imposed curfew, as well as a prohibition on gatherings and weapons.
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