DABIQ, Syria - A Qatari mission has begun searching for the remains of US hostages killed by ISIS a decade ago, two sources briefed on the mission said, reviving a longstanding effort to recover their bodies.

ISIS, which controlled swathes of Syria and Iraq at the peak of its power from 2014 to 2017, beheaded numerous people in captivity, including Western hostages, and released videos of the killings.

Qatar's international search and rescue group began the search on May 7, accompanied by several Americans, the sources said.

The group, deployed by Doha to earthquake zones in Morocco and Turkey in recent years, had so far found the remains of three bodies, the sources said.

One of the sources - a Syrian security source - said the remains had yet to be identified. The second source said it was unclear how long the mission would last.

The US State Department had no immediate comment.

The Qatari mission gets under way as US President Donald Trump prepares to visit Doha and other Gulf Arab allies next week and as Syria's ruling Islamists, close allies of Qatar, seek relief from US sanctions.

The Syrian source said the mission's initial focus was on looking for the body of aid worker Peter Kassig, who was beheaded by ISIS in 2014 in Dabiq in northern Syria. The second source said Mr Kassig's remains were among those they hoped to find.

US journalists James Foley and Steven Sotloff were among other Western hostages killed by ISIS. Their deaths were confirmed in 2014.

US aid worker Kayla Mueller was also killed in ISIS captivity. She was raped repeatedly by Islamic State leader Abu Bakr al-Baghdadi before her death, US officials have said. Her death was confirmed in 2015.

"We’re grateful for anyone taking on this task and risking their lives in some circumstances to try and find the bodies of Jim and the other hostages," said Ms Diane Foley, James Foley's mother. "We thank all those involved in this effort."

The families of the other hostages, contacted via the Committee to Protect Journalists, did not immediately respond to a request for comment.

The jihadists were eventually driven out of their self-declared caliphate by a