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

ISIS, which controlled swathes of Syria and Iraq at the peak of its power from 2014-2017, 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 Wednesday, accompanied by several Americans, the sources said. The group, deployed by Doha to earthquake zones in Morocco and Türkiye 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 Saudi Arabia, the United Arab Emirates and Qatar next week and as Syria's new rulers 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 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. 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 Diane Foley, James Foley's mother. "We thank all those involved in this effort."

Two ISIS members, both former British citizens who were part of a cell that beheaded American hostages, are serving life prison sentences in the United States.