US Attorney General William Barr told Britain that two former ISIS militants accused of kidnapping and killing Americans would not face execution if tried in the US, the Justice Department said on Wednesday.
Mr Barr told British Interior Minister Priti Patel in a letter that the Justice Department would not pursue the death penalty against Alexanda Kotey and El Shafee Elsheikh.
The two were part of an ISIS cell called "The Beatles" and have since been stripped of British citizenship.
"If imposed" by a court, "the death penalty will not be carried out", Mr Barr said in the letter.
He made the pledge to gain access to British evidence against the two, who are being held in Iraq by the US military.
The UK, which has not moved to put the two on trial, does not have capital punishment and a British court prevented co-operation in the case if they were to face possible execution.
It is believed that intervention by the family of victim Kayla Mueller, a US activist who was kidnapped in Aleppo in 2013 and killed two years later, spurred Mr Barr's decision to make the compromise.
"If a prosecution is to go forward in the United States, our prosecutors should have the important evidence that we have requested from the United Kingdom available to them in their efforts to hold Kotey and Elsheikh responsible for their terrorist crimes," he wrote.
"We would hope and expect that, in light of this assurance, the evidence can and will now be provided promptly."
Kotey and Elsheikh were part of a four-member ISIS kidnapping gang called "The Beatles" because of their heavy British accents.
They were notorious for videotaping beheadings and allegedly killed US journalist James Foley eight years ago on Friday, and western aid workers.
The other members were Mohammed Emwazi, known as "Jihadi John", who was killed in an air strike in 2015, and Aine Davis, who has been jailed in Turkey.