The so-called Islamic State of Iraq and the Levant (ISIL) terrorist group massacred more than 700 members of a tribe in Eastern Syria in two weeks, the Syrian UK-based Observatory said.
Among the members of the Shaitat tribe killed were 100 fighters, but the rest were civilians, the monitoring group said, Al-Manar reported.
They were killed in several villages in the Eastern province of Deir Ezzur.
They were killed in the Ghranij, Abu Hamam and Kashkiyeh villages of the mainly ISIL-controlled province of Deir Ezzur, said the Observatory, which relies on a vast network of activists and medics on the ground for its information.
The conflict in Syria started in March 2011, when sporadic pro-reform protests turned into a massive insurgency following the intervention of western and regional states.
The unrest, which took in terrorist groups from across Europe, the Middle-East and North Africa, has transpired as one of the bloodiest conflicts in recent history.
As the foreign-backed insurgency in Syria continues without an end in sight, the US government has boosted its political and military support to Takfiri extremists.
Washington has remained indifferent to warnings by Russia and other world powers about the consequences of arming militant groups.