The Syrian Observatory for Human Rights said on Wednesday that ISIS 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 ISIS had been holding in territory near the Iraqi border.
The extremists control a shrinking strip of land east of the Syria's Euphrates River around the town of Hajin, which US-backed forces entered this month.
The Syrian Democratic Forces (SDF), led by the Kurdish YPG militia, has battled ISIS there for several months with the help of US air power and special forces.
SDF commander-in-chief Mazloum Kobani told Reuters last week that at least 5,000 IS fighters remain holed up in the enclave, including many foreigners who appear ready to fight to the death.
ISIS' 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.
