According to officials quoted by US media, Alexanda Amon Kotey, 34, and El Shafee Elsheikh, 29, were captured by the Syrian Democratic Forces, operating south of the Euphrates river, close to the Syria-Iraq border in early January and US forces were given access to them.
Kotey and Elsheikh, along with their ringleader Mohammed Emwazi or Jihadi John and Aine Davis, were part of the IS' insurgent cell which was nicknamed "The Beatles", the UK's famous four-member pop group, because of their British accents.
Kotey, a convert to Islam, grew up in west London, and Elsheikh's family fled to the UK from Sudan in the 1990s.
Their capture was first reported by 'The New York Times', with the UK Foreign Office saying, it does not "comment on individual cases or ongoing investigations".
More From This Section
According to a US State Department's terrorist designation, Kotey operated as a guard for "The Beatles" cell and "likely engaged in the group's executions and exceptionally cruel torture methods, including electronic shock and waterboarding".
The families of the men told the BBC they were not aware of the capture until the story was reported by news organisations.
John, the group leader, was believed to be the masked militant responsible for gruesome beheadings of journalists James Foley and Steven Sotloff, which were captured on film and distributed in ISIS propaganda videos.
He was killed in a drone strike in the former ISIS stronghold of Raqqa in November 2015.
UK defence secretary Gavin Williamson told 'The Times': "These are people who have done absolutely vile and despicable crimes and brought absolutely so much misery. It is good that they have been hunted down and caught."
The group of ISIS operatives became known for their alleged brutalityand the US government has claimed that they were involved in the beheading of more than 27 hostages.