The Syrian Observatory for Human Rights said on Wednesday that Islamic State militants had executed nearly 700 prisoners in nearly two months in eastern Syria.
The UK-based war monitoring group said the prisoners were among 1,350 civilians and fighters that Islamic State had been holding in territory near the Iraqi border.
The jihadists control a shrinking strip of land east of the Syria's Euphrates River around the town of Hajin, which U.S.-backed forces entered this month.
The Syrian Demoratic Forces (SDF), led by the Kurdish YPG militia, has battled Islamic State there for several months with the help of U.S. air power and special forces.
SDF commander-in-chief Mazloum Kobani told Reuters last week that at least 5,000 ISIS fighters remain holed up in the enclave, including many foreigners who appear ready to fight to the death.
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Islamic State's self-proclaimed caliphate has crumbled after different offensives across Iraq and Syria, though its fighters still operate in the desert border region and mount attacks.