Human Rights Watch called Tuesday for Damascus and the Kurdish authorities in northeastern Syria to investigate the fate of thousands of people who went missing in the custody of the Islamic State group.

More than 8,000 people, detained by IS when it controlled swathes of the north and east, remain unaccounted for, the New York-based watchdog said, citing figures from the Syrian Network for Human Rights.

Among them are a few well-known cases of foreigners, such as British reporter John Cantlie and Italian Jesuit priest Paolo Dall'Oglio.

HRW said the Syrian government and the Kurdish authorities who now control former IS territory have so far failed to prioritise the search for the truth about what happened to the missing.

Kurdish authorities in the northeast "rarely" provide answers to those in search of their relatives, HRW said.

"People whose relatives had been in areas now under the control of Syrian government forces similarly said they received only speculation that ISIS killed all its captives, or blanket denials of any knowledge," the report said, using an alternative acronym for IS.

More than 8,000 people last seen in the custody of the Islamic State group remain unaccounted for in Syria despite the exhumation of mass graves, like this one near the jihadists' former stronghold of Raqa
More than 8,000 people last seen in the custody of the Islamic State group remain unaccounted for in Syria despite the exhumation of mass graves, like this one near the jihadists' former stronghold of Raqa AFP / Muhammad AHMAD

US-backed Kurdish-led forces overran the jihadists' last pocket of Syrian territory in the eastern village of Baghouz in March last year.

It was the culmination of rival campaigns by the Kurdish authorities and the Russian-backed government in Damascus to claw back the territory IS seized in 2014.

"The end of the territorial control of ISIS provides an opportunity to deliver answers to families whose relatives went missing in Syria," said HRW deputy regional director Joe Stork.

The rights group said the Kurdish authorities should create a "centralised focal point or civilian body" to collect information about the missing and reach out to their families.

They should also dedicate resources to protecting the mass graves believed to contain the bodies of thousands of people executed by IS, it added.

Dozens of such mass graves have been found in Iraq and Syria but the identification process is slow, costly and complicated.

The Kurdish administration lacks the official status to appeal for international aid and its own resources are hard pressed coping with the aftermath of IS's territorial defeat and mounting pressure from a hostile Turkey.

Iraq