The fate of two suspected members of the notorious 'IS Beatles' execution cell risks sparking a row with the US as British authorities insist they should not take responsibility for hosting the men's trial.
Londoners Alexanda Kotey and El Shafee Elsheikh are thought to be the final two members of the murderous gang which beheaded a number of western hostages.
The US Pentagon chief Jim Mattis is expected to use a meeting today with Defence Secretary Gavin Williamson in Rome to insist the men should face trial in the UK.
But British authorities - who previously stripped the pair of their UK citizenship - are so far adamant they they should not be allowed back on British soil.
There are some suggestions that Kotey and Elsheikh could be tried at the International Criminal Court in The Hague.
Another option is for the pair to be sent to the US detention facility Guantanamo Bay - though this would be controversial and Washington is resisting this option.
Kotey and Elsheikh were captured by Kurdish militia fighters inside Syria in January, and are currently under guard in the caliphate's former heartland.
Authorities in Washington want all militants captured by the coalition-backed Syrian Democratic Forces to be turned over to face justice in their home countries.
Mr Mattis is expected to press that point at today's meeting of ministers from the international coalition fighting IS.
There are also some calls within Britain to bring to two men to the UK for trial.
Lord Carlile, a former independent reviewer of terrorism legislation, had said that a British trial was the "proper forum" for justice.
But Mr Williamson has made it clear that he does not want Kotey and Elsheikh to be allowed back into the UK under any circumstances.
Her comment comes after Justice Secretary David Gauke told Peston on Sunday the government was still considering what its options.
Home Secretary Amber Rudd has remained tight-lipped over the row, saying that the pair must "face justice" but refusing to comment ton the venue for their trial.
"We will always do what is right to make sure that we protect British people and that they get the justice they deserve," she said.
"I'm not going to comment any further on where that should be."
US authorities said they are also considering their options as regards to the two men.
Kotey and Elsheikh are thought to have been in IS gang together with Mohammed Emwazi - also known as Jihadi John - and Aine Davis.
They were dubbed "the Beatles" because of their English accents.
Emwazi, who appeared in horrific videos showing the beheading of British and American hostages, was killed in a British drone strike in 2015.
Davis is in custody in Turkey where he was convicted on terrorism charges.