The Syrian Observatory for Human Rights said the Islamic State (IS) jihadists have carried out the killings over the past two weeks in oil-rich Deir Ezzor province which the group mostly controls.
Among the members of the Shaitat tribe killed were 100 fighters, but the rest were civilians, the Britain-based monitoring group said.
They were killed in Ghranij, Abu Hamam and Kashkiyeh villages, said the Observatory, which relies on a vast network of activists and medics on the ground for its information.
Observatory head Rami Abdel Rahman said the fate of 1,800 other members of the Sunni Muslim tribe was unknown.
Fighting between the jihadists and the tribe erupted after a deal between them collapsed, with the Shaitat refusing to bow to IS authority.
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 IS, in exchange for the jihadists not harassing or attacking its members.
But the IS had detained three members of the tribe, "violating" the agreement.
In northern Syria, rebels were yesterday defending one of their main strongholds near the Turkish border against a rapid IS advance, the Observatory and an activist said.
The opposition Syrian National Coalition (SNC), meanwhile, called for the US military to carry out air strikes on IS positions not only in northern Iraq but also in Syria.
IS fighters were nearing the towns of Marea and Aazaz, held by rebel groups battling Syrian President Bashar al-Assad's regime, after seizing a number of villages in Aleppo province.
"The pressure is now on Marea, militarily," said activist Abu Omar, spokesman for the town's "revolutionary council".
"The rebels have sent in many reinforcements and weapons to the area in and around Marea," he told AFP, speaking via the Internet.
"The rebels consider this to be one of the most important battles against the IS... There's no question of losing," he added.
Abu Omar said the jihadist group, which is also currently being targeted by US air strikes as it battles Western-backed Kurdish forces in northern Iraq, is using heavy weaponry it seized when it routed the Iraqi army in June.

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