The two men, El Shafee Elsheikh and Alexanda Amon Kotey, were part of an IS terror cell that has kidnapped and murdered hostages. The four men were nicknamed The Beatles for their recognisable English accents.
Elsheikh and Kotey were captured in 2018 trying to flee the region, and have been in a Syrian prison since then.
Earlier, US officials had said that the pair were moved out of Syria by military and law enforcement personnel to make sure that they could not escape as a result of the breakdown of security following the Turkish invasion in the country.
A US official said the pair had been moved to Iraq "out of an abundance of caution", adding that the US was still committed to bringing them to justice.
Speaking to Sky News in 2018, the pair warned that IS were planning new attacks in Europe, and refused to condemn the terror attacks in London and Manchester.
The terror cell beheaded journalists from the UK, US and Japan, as well as aid workers and Syrian soldiers in videos released by IS to the world.
