A Qatari mission has begun searching for the remains of U.S. hostages killed by Islamic State in Syria a decade ago, two sources briefed on the mission said, reviving a longstanding effort to recover their bodies.

Islamic State, 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 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 U.S. State Department had no immediate comment.

The Qatari mission gets under way as U.S. 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 U.S. 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 Islamic State in 2014 in Dabiq in northern Syria. The second source said Kassig's remains were among those they hoped to find.

U.S. journalists James Foley and Steven Sotloff were among other Western hostages killed by Islamic State. Their deaths were confirmed in 2014.

U.S. aid worker Kayla Mueller was also killed in Islamic State captivity. She was raped repeatedly by Islamic State leader Abu Bakr al-Baghdadi before her death, U.S. 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 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 U.S.-led coalition and other forces.

APRIL VISIT

Plans for the Qatari mission were discussed during a visit to Washington in April by Qatari Prime Minister Sheikh Mohammed bin Abdulrahman Al Thani and the Minister of State for the foreign ministry, Mohammed Al Khulaifi - a trip also designed to prepare fo...