Britain is in talks with the US about how to deal with two suspected IS fighters.
Alexanda Kotey and El Shafee Elsheikh are both from west London and were detained in Syria last month on suspicion of terrorist activity.
There have been disagreements between the UK and US on who should try the pair.
The Home Secretary, Amber Rudd, says it's still not clear where they'll stand trial.
However, she said that she was "absolutely convinced and absolutely committed" to the idea of Alexanda Kotey and El Shafee Elsheikh "facing justice" after their capture in Syria in January.
But she declined to answer when asked by the Press Association where the men, alleged to be the only living members of a British-accented quartet of suspected killers, might be tried.
Last week Defence Secretary Gavin Williamson said that the pair should not return to the UK because they had "turned their back on British ideas, British values".
His comments threatened to spark a diplomatic row with the United States, which wants countries to take responsibility for their citizens arrested in the fight against IS.
Ms Rudd, who was speaking on a visit to the Middle East, said: "The important thing is that they have been arrested.
"And the important thing is, to us, that they will face trial."
"I can't be drawn at the moment on where that will be, but I am absolutely convinced, and absolutely committed, to making sure that they will face trial because the security of the United Kingdom will always come first."
Opinion is currently divided on where and how the men should be tried.
It is understood that the pair has been stripped of their British citizenship, although officials at the Home Office have refused to comment on individual cases.
Mr Williamson, speaking in Brussels, said: "Do I want them back in the United Kingdom? No, I don't."
His comments came after talks with his US counterpart Jim Mattis, where the Pentagon chief urged countries to take back their fighters, saying:
"The important thing is that the countries of origin keep responsibility for them."
US officials have also said putting the two in Guantanamo Bay is not an option.
Junior defence minister Tobias Ellwood has suggested the men should be tried at the International Criminal Court in The Hague.
But Lord Carlile, the UK's independent reviewer of terrorism legislation from 2001 to 2011, said a British trial was the "proper forum".