The war monitor said it has received “confirmations” from security sources within the Islamic State group and from residents who have recently fled areas under its control.
“Credible sources confirmed to the Syrian Observatory that the Islamic State organization executed more than 700 of its prisoners,” the Observatory said.
Some of those killed were ISIS deserters, the monitor said.
The executions took place in the group’s headquarters, its prisons, and in the remaining areas under its control, the SOHR said. The dead are thought to have been buried in mass graves.
Russian President Vladimir Putin told a summit in the Black Sea resort of Sochi in October that ISIS was holding 700 captives, including Syrians, Americans, and Europeans.
The US-backed Syrian Democratic Forces (SDF) announced on Friday it had ousted ISIS from its largest remaining stronghold of Hajin in Deir ez-Zor province.
Sporadic clashes are still ongoing as the group maintains control of a small pocket of territory on the east bank of the Euphrates, including the towns of Shafaa, al-Sousah, and al-Baghouz.
ISIS also controls desert areas on the west bank of the Euphrates, where it has reportedly transferred 350-400 of its prisoners.
According to SOHR, ISIS is known to have executed 6,191 prisoners on Syrian soil since 2014, often in a brutal fashion. Several executions were filmed and posted on the internet in propaganda videos, including the burning alive of a captured Jordanian pilot.
SOHR implored the US-led Coalition and the SDF to interrogate ISIS security officials to discover the whereabouts of mass graves and the fate of missing people.
Around 3,000 Yezidi women and girls are still unaccounted for, while 40 Kurdish Peshmerga fighters captured in battle are missing.
The fate of several Westerners, including journalists Austin Tice and John Cantlie, is still unknown.
