What to do with hundreds of foreign so-called Islamic State (IS) 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.

A senior administration official said on Tuesday that resolving the fate of these prisoners is a top priority as the government lays the groundwork with allies to comply with President Donald Trump's December 19 order to withdraw the 2,000 American troops from Syria, where they have been working alongside the US-backed Syrian Defence Forces to fight the IS since 2015.

But there are no easy answers. The official, speaking on condition of anonymity, said releasing the fighters, among them Europeans and some US citizens, would be “unacceptable” since they could simply rejoin the remnants of IS fighters 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 the IS, not wanting the legal challenge of prosecuting them or the potential security risk if they are released.

And moving former fighters to the United States poses some of the same challenges the US has faced with men detained at the military base at Guantanamo Bay, Cuba, including 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.

Meanwhile, the prisoner problem is only growing worse.

On Sunday, the Syrian Defence Forces announced the capture of five fighters, including two US citizens, one of whom has been identified as a former school teacher from Houston.

Secretary of State Mike Pompeo just began a tour of eight Middle Eastern nations to discuss the withdrawal of the American troops. National security adviser John Bolton returned Tuesday from a meeting in Turkey, where he was seeking a guarantee of safety for the Kurdish fighters who have fought alongside US troops against the IS.

There are fears that the US withdrawal will leave a door open for Turkey to assault the US-allied 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 US withdrawal.

Administration officials so far do not have a plan for what to do with the prisoners, according to a separate US official, who said that few countries have been willing to accept any of their captured citizens. Both officials spoke on condition of anonymity because they weren't authorised to disclose the information publicly.

In a recent case of an American suspected of IS membership, US 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 2,000 US troops from Syria. When he made his surprise announcement of the withdrawal three weeks ago, Trump said he wanted to complete it quickly. His abrupt decision led to the resignations of Defence Secretary Jim Mattis and Brett McGurk, special presidential envoy for the global coalition to defeat IS.

More recently, Trump and other administration officials have insisted they favour an orderly pullout.

The senior administration official said the US will defeat remaining IS fighters on the way out to prevent a resurgence and that the US will oppose any mistreatment of opposition forces, such as the SDF, that fought with the United States against IS.

“These questions are hard enough, if you know the timeline on which you're making them if you know what the US involvement will or won't be over that timeline,” Geltzer said.

One of the foreign fighters recently captured is Warren Christopher Clark, a former substitute schoolteacher from Houston, Texas, 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 IS out of the around 295 whom intelligence officials claim have travelled 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, Zaid Abed al-Hamid, 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 IS,” 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.”