It isn't clear what countries those prisoners are from beside Syria.
Sputnik News previously reported that The Syrian Observatory for Human Rights is a one-man war monitoring shop operated by rebel-sympathizer Rami Abdulrahman and funded by the UK Home Office.
According to the monitor, Daesh holds around 1,350 civilian and military prisoners near the Iraqi border. Among those were the ones executed recently.
US-backed Kurdish forces entered the town of Hajin, which is on the Iraqi border east of Deir ez-Zor along the Euphrates river, where Daesh maintains its last stronghold in Syria. Across the border, Daesh maintains its last presence in Iraq in a desert region north of Rawa, according to a live map of the conflict.
Last week, the commander of the US-backed Syrian Democratic Forces told Reuters that at least 5,000 Daesh fighters are still in the eastern enclave around the Euphrates in Syria, many of whom are prepared to fight to the death.
On Wednesday morning, US President Donald Trump tweeted that "we have defeated ISIS," which he said was his "only reason for being" in Syria.
Separate statements from the White House and the Pentagon claimed that the US was embarking on a process of withdrawing its troops from the country.