Gavin Williamson has fuelled a diplomatic row with the United States by insisting that the UK will not take back the British Islamic State suspects dubbed the Beatles.
Washington has said that countries should take responsibility for their citizens captured in the battle against IS.
But Defence Secretary Mr Williamson has 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.
Mr Williamson, speaking in Brussels, said the two – alleged to be part of a notorious IS terror cell known as the Beatles because of their British accents – should not return to the UK.
They had “turned their back on British ideas, British values”, he said.
“Do I want them back in the United Kingdom? No, I don’t.”
Mr Williamson held talks with US counterpart Jim Mattis on Wednesday but declined to say what possible solutions they had discussed.
Both men had been at an international summit in Rome on Tuesday where the Pentagon chief urged countries to take back their fighters.
Mr Mattis said “the important thing is that the countries of origin keep responsibility for them”.
It is understood that the pair have been stripped of their British citizenship, although officials at the Home Office have refused to comment on individual cases.
A Whitehall source said: “They are not British subjects and should pay the price for their crimes in Syria.”