The fate of two British men suspected of being members of an Islamic State execution group dubbed "The Beatles" could escalate into a diplomatic row between the UK and United States.
Washington wants militants captured by the coalition-backed Syrian democratic forces to be turned over to face justice in their home countries.
But British ministers have resisted suggestions that Alexanda Kotey and El Shafee Elsheikh should return to the UK to stand trial.
The two men were captured in Syria, having fallen into the hands of Kurdish militia fighters in January, and are under guard in the caliphate's former heartland.
Defence Secretary Gavin Williamson and US counterpart Jim Mattis will attend a meeting of ministers from the coalition fighting IS in Rome on Tuesday.
The Pentagon chief is expected to raise the issue of how to deal with captured militants during the talks.
Mr Williamson has said he does not think the so-called Beatles "should ever set foot in this country again".
Junior defence minister Tobias Ellwood has suggested the men should be tried at the International Criminal Court in The Hague.
Justice Secretary David Gauke said on Sunday the Government was considering the "practical options in these circumstances".
US officials have said putting the two in Guantanamo Bay is not an option.
Kathryn Wheelbarger, the US principal deputy assistant defence for international security affairs, said: "We're working with the coalition on foreign fighter detainees, and generally expect these detainees to return to their country of origin for disposition."
The Syrian democratic forces are capturing up to 40 fighters a day and their facilities are "eventually going to be full", she said.