ROME (Reuters) - As U.S.-backed militias in Syria capture a growing number of Islamic State militants, the United States wants to see foreign fighters returned to their countries of origin for prosecution, a senior Pentagon official said on Sunday.

Abu Malek, one of the survivors of a chemical attack in the Ghouta region of Damascus that took place in 2013, uses his crutches to walk along a street in the Ghouta town of Ain Tarma, Syria, April 7. REUTERS/Bassam Khabieh

The question of how to deal with captured foreign fighters received renewed attention when U.S. officials said the Syrian Democratic Forces (SDF) - an alliance of militias dominated by the Kurdish YPG - had captured two of four militants known as the “Beatles” for their English accents.

A U.S. official, speaking on condition of anonymity, said the United States was in talks with the British about what to do with them but at this point there were no plans to bring them to the United States or Guantanamo Bay.

About a dozen members of the U.S.-led coalition fighting Islamic State will meet in Rome this week, where the United States, led by Defense Secretary Jim Mattis, will emphasize the need for countries to take back foreign Islamic State detainees.

“We are working with the coalition on foreign fighter detainees, and generally expect these detainees to return to their country of origin for disposition,” Katie Wheelbarger, the principle deputy assistant secretary of defense for international security affairs, said.

U.S. officials have expressed concerns about the lack of a clear path on how to deal with foreign fighters detained by the SDF.

The large number of detainees mixing in detention facilities often crammed with prisoners, could lead to a proliferation of militant views and deeper radicalization, officials have said.

The other concern is SDF detention facilities reaching capacity. At one point, SDF forces were capturing 40 to 50 Islamic State fighters, including Syrians, a day.

U.S. officials say there are hundreds of foreign fighters and thousands of Syrian Islamic State militants in SDF hands.

“The capacity problem is very real ... I think they are willing to hold them as long as we need,” Wheelbarger said.

“(But) if they continue to capture them at the rates that they are, their facilities are eventually going to be full.”