The US State Department on Monday called on countries to take in foreign fighters captured by America's Kurdish allies in Syria, two days before a meeting in Washington of dozens of coalition partners fighting ISIS to discuss the way forward in Syria.
The department's spokesman, Robert Palladino, did not say how many prisoners had been detained by the Syrian Democratic Forces — an alliance of militias dominated by the Kurdish YPG — but US officials believe they number about 850.
The United States has long said it wants to see the fighters returned to their countries of origin in Europe, the Middle East and North Africa to face prosecution.
"The SDF has demonstrated a clear commitment to detain these individuals securely and humanely," Mr Palladino said in a statement, which gave no details of where the prisoners were being held.
"The United States calls upon other nations to repatriate and prosecute their citizens detained by the SDF," he said.
US officials say there has been a slight sharp rise in the number of foreign fighters in SDF custody as US-backed forces retake the last remaining ISIS-held areas.
The issue is likely to be raised on Wednesday at the first high-level meeting of allies since President Donald Trump's announcement in December that he was pulling US forces from Syria.
Mr Trump's announcement stunned allies, especially the SDF, which has been among the most effective against ISIS, and led to the resignation of Defence Secretary Jim Mattis.
A Pentagon adviser, speaking on condition of anonymity due to the sensitivity of the issue, said the State Department statement reflected the dilemma about what to do with the prisoners created by the withdrawal of US forces.
"This was a problem created by surprise with an ill-co-ordinated and ill-considered announcement of complete US withdrawal from Syria," said the Pentagon adviser. "We now have precipitated a crisis because our allied counties were not expecting to have to bring back home their foreign fighters."
A senior State Department official, speaking to reporters before Wednesday's meeting, said the administration's goals in Syria remained the defeat of ISIS, the exit of all Iranian forces from Syria and reaching a political settlement to end the conflict.
The official said the meeting was not a pledging conference, although he emphasised there were funding shortfalls for humanitarian assistance in northeast Syria and Iraq, as well as efforts at mine removal and stabilisation.