Alexanda Kotey and El Shafee Elsheikh were detained by US-allied Kurdish militia fighters in January, the New York Times reported.
Along with Mohammed Emwazi – the killer nicknamed Jihadi John – and Aine Davis, they are believed to have been part of a group named after The Beatles because of their English accents.
Unnamed US Officials told the NYT that Kotey and Elsheikh were captured by the Kurdish-led Syrian Democratic Forces which were fighting the last remaining pockets of IS fighters near the river Euphrates on the Iraq/Syria border.
It added that the men were identified by fingerprints and other biometric means.
Former child refugee Elsheikh was a mechanic from White City in west London, and Alexanda Kotey was from Paddington.
In January 2017, US authorities named Kotey as a member of the cell and said they had imposed sanctions on him.
Emwazi, who was killed in a US air strike in 2015, appeared in a number of videos in which captives including British aid workers David Haines and Alan Henning were beheaded.
The fourth member, Davis, was convicted of being a member of a terrorist organisation and jailed for seven-and-a-half years at a court in Silivri, Turkey, in May 2017.
Source: Evening Standard