The Islamic State group has executed 700 members of a tribe it has been battling in eastern Syria during the past two weeks, the majority of them civilians, a monitoring group said.
The killing spree happened in several villages inhabited by the al-Sheitat tribe in Deir Ezzor province, where the tribe are from, the Syrian Observatory for Human Rights said on Saturday.
The silence of the international community is unbelievable. There is no excuse for them to keep a blind eye on what is happening in Syria.
The Observatory said many of the victims, who were Sheitat tribesmen, were beheaded after they were captured by the Islamic State group.
Among the members of the Sheitat tribe killed were 100 fighters, but the rest were civilians, the activist group, which opposes Syrian President Bashar al-Assad, said.
The men were killed in the Ghranij, Abu Hamam and Kashkiyeh villages of the mainly IS-controlled province of Deir Ezzor, according to the Observatory, which relies on a vast network of activists and medics on the ground for its information.
Observatory head, Rami Abdel Rahman, said that the fate of 1,800 other members of the tribe was unknown.
On Saturday, the opposition Syrian National Coalition (SNC) called on the United States and its allies to conduct air strikes in Syria against the Islamic State group, similar to those being carried out against the same group's positions in neighbouring Iraq.
During a press conference in Turkey SNC leader, Hadi al-Bahra, accused the international community of double standards.
"The silence of the international community is unbelievable," he said. "There is no excuse for them to keep a blind eye on what is happening in Syria. We have reports supported by documents and videos that crimes in Deir Ezzor against humanity are being committed by IS on a daily basis".
Gruesome videos have surfaced online purporting to be of the beheadings in several Sheitat villages. The footage showed men laughing and mocking the victims by mimicking goats as they performed the executions.
Some of those killed were said to be injured men who had fought against the Islamic State group. Reports suggested IS fighters dragged the men from the nearby Hujein hospital and the New Medical Center in neighbouring Mayadeen City before cutting off their heads.
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The killings happened after failed negotiations between the two sides, and after Sheitat tribal elders openly refused to pledge allegiance to the Islamic State.
Last Sunday, one Sheitat leader urged all IS fighters to repent to God and abandon the group. He called on IS fighters to follow "the religion of mercy" and abandon what he described as "the cattle of the deviant".
The tribesmen then killed some IS fighters and displayed their bodies in Sheitat.
In a separate development on Saturday, at least 22 people were killed when a car bomb went off in a southern Syrian province controlled by armed rebels fighting Assad.
The explosion on Saturday happened in front of a mosque in the rebel-controlled town of Namar in Deraa Province.
Activists told Al Jazeera that the casualties included women, and at least one child was among the dead.