Islamic State (IS) militants were facing charges of a new massacre after a human rights monitoring group and activists alleged the group executed 700 members of a tribe it has been battling in eastern Syria during the past two weeks, the majority of them civilians.
This comes as Syria's Western-backed opposition called on Saturday for US air strikes against Islamic State, which overran nearly a dozen towns and villages in Aleppo province last week.
The Syrian Observatory for Human Rights said Islamic State jihadists had carried out the killings over the past two weeks in oil-rich Deir-ez-Zor province which the group mostly controls.
Among members of the Shaitat tribe killed were 100 fighters, but the rest were civilians, the Britain-based monitoring group said.
They were killed in the villages of Ghraneij, Abu Hamam and Kashkiyeh, said the Observatory, which relies on a vast network of activists and medical workers on the ground for its information. An activist in Deir-ez-Zor said 300 men had been executed in one day in the town of Ghraneij.
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.
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 said.
At a news conference in Gaziantep in southeast Turkey, the leader of the Syrian National Coalition, Hadi al-Bahra, accused the West of double standards.
"In the name of humanity, I call on the United Nations and all countries which believe in freedom, starting with the United States, to take action in Syria in the same way it did in Iraqi Kurdistan," he said.
"The causes are the same, the enemy is the same and double standards should not apply," said the leader of the exiled opposition group.
The Marea battle comes after IS fighters took control of around 10 nearby villages on Wednesday and Thursday, the Observatory said.
It said the fate of dozens of rebels captured during the jihadist offensive remained unknown after nine were beheaded on Wednesday and another eight on Saturday in Akhtarin.
Taking the towns of Marea and Aazaz would cut supply lines to rebel groups.
Aazaz, next to a border crossing with Turkey, would be a valuable asset for IS as it seeks to expand its self-declared "caliphate" in the territories it holds in Syria and Iraq.
Agence France-Presse, Reuters, Associated Press