FILE - In this July 21, 2017, file photo, Kurdish soldiers from the Anti-Terrorism Units, carry a blindfolded an Indonesian man suspected of Islamic State membership, at a security center, in Kobani, Syria. What to do with hundreds of foreign Islamic State fighters captured in Syria has become a critical and growing problem for the Trump administration as it prepares to pull troops out of the country (AP Photo/Hussein Malla, File)

FILE - In this July 21, 2017, file photo, Kurdish soldiers from the Anti-Terrorism Units, carry a blindfolded an Indonesian man suspected of Islamic State membership, at a security center, in Kobani, Syria. What to do with hundreds of foreign Islamic State fighters captured in Syria has become a critical and growing problem for the Trump administration as it prepares to pull troops out of the country (AP Photo/Hussein Malla, File)

WASHINGTON (AP) — Hundreds of foreign Islamic State fighters have been captured in Syria, and what to do with them has become a problem for the Trump administration as it prepares to pull American troops from the country.

An administration official said determining the fate of these prisoners, who include Europeans and some U.S. citizens, is a priority as the U.S. lays the groundwork with allies to comply with President Donald Trump’s order to withdraw the 2,000 American troops from Syria. Those troops have worked alongside the U.S.-backed Syrian Defense Forces to fight IS since 2015.

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The official, who was not authorized to publicly discuss private deliberations about policy and spoke on condition of anonymity, said Tuesday that releasing the fighters would be “unacceptable” because of concerns that they could rejoin IS militants in Syria or elsewhere.

“This matters because SDF holds hundreds of IS fighters, including many European citizens, and they might go free if no solution is found,” said Bobby Chesney, a national security law expert at the University of Texas.

European nations have been reluctant to take back citizens with ties to IS, not wanting the legal challenge of prosecuting them or the potential security risk if they are released.

Moving former fighters to the United States poses some of the same challenges that Washington has faced with men detained at the military base at Guantanamo Bay, Cuba. Among the issues are whether it’s feasible to prosecute militants captured on the battlefields of northern Syria, according to experts.

“It’s one thing for the government to be very confident that an individual joined or tried to join ISIS. And sometimes it’s still another thing for the government to be able to mount confidently a criminal prosecution against that individual,” said Joshua Geltzer, a senior counterterrorism official under President Barack Obama.

A Syrian Kurdish militia said Wednesday it had captured eight IS fighters, including a 16-year-old American, a German, and a Russian. On Sunday, the SDF announced the capture of five fighters, including two U.S. citizens.

Secretary of State Mike Pompeo is visiting the Mideast to discuss the withdrawal of the American troops from Syria. National security adviser John Bolton returned Tuesday from a meeting in Turkey, where he sought a guarantee of safety for the U.S.-allied Kurdish fighters in Syria.

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There are fears that the U.S. withdrawal will leave a door open for Turkey to assault the SDF fighters. Turkey views them as part of a terrorist group linked to an insurgency within its own borders. SDF commanders have warned that they will be unable to hold the 700 prisoners if Turkish forces invade Syria following a U.S. withdrawal.

Administration officials so far do not have a plan for what to do with the prisoners, according to a second U.S. official, who said that few countries have been willing to accept any of their captured citizens. This official was not authorized to disclose the information publicly and spoke on condition of anonymity.

In a recent case of an American suspected of IS membership, U.S. officials wrestled for more than a year, never charged him and then ultimately released him in Bahrain.

The problem has been further complicated by conflicting reports of Trump’s timeline for recalling the U.S. troops from Syria.

One of the foreign fighters recently captured is Warren Christopher Clark, a former substitute schoolteacher from Houston, who was first identified by George Washington University’s Program on Extremism. Researchers spent months investigating to confirm his identity through multiple sources. The program has identified more than 73 Americans, by their legal names, who are known to have joined jihadist groups.

“Clark is one of several dozen Americans to join the Islamic State out of the around 295 whom intelligence officials claim have traveled or attempted to travel to Syria and Iraq to join the terrorist group,” according to Seamus Hughes, deputy director of the Program on Extremism.

Clark was captured along with four other foreign jihadists — two from Pakistan, one from Ireland and a fourth man, who also is believed to be from the United States, although that has not been confirmed.

In a letter to IS that was obtained by the researchers, Clark submitted a resume noting that he had a bachelor’s degree from the University of Houston, had worked as a substitute teacher at the Fort Bend Independent School District in Sugar Land, Texas, and had done teaching stints in Saudi Arabia and Turkey.

“Dear Director, I am looking to get a position teaching English to students in the Islamic State,” he wrote to the group in an accompanying cover letter. “Teaching has given me the opportunity to work with people from diverse cultural backgrounds and learning capabilities.”

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Associated Press writer Zeke Miller contributed to this report.