US Attorney General Bill Barr told Britain that two former Islamic State militants accused of kidnapping and killing Americans would not face execution if put on trial in the United States, the Justice Department said Wednesday.

Barr told Home Secretary Priti Patel in a letter Tuesday that the Justice Department would not pursue the death penalty against Alexanda Kotey and El Shafee Elsheikh, British citizens who were part of an IS kidnapping unit dubbed "The Beatles" and have since been stripped of their citizenship.

"If imposed" by a court, "the death penalty will not be carried out," he said in the letter.

Barr made the pledge in order to gain access to British evidence against the two, who are being held in Iraq by the US military.

Britain, which has not moved to put the two on trial itself, does not have capital punishment, and a British court had prevented cooperation in the case if they were to face possible execution in the United States.

"If a prosecution is to go forward in the United States," Barr wrote, "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," Barr 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 kidnapping gang within the Islamic State group dubbed "The Beatles" by their captives due to their heavy British accents.

They were notorious for videotaping beheadings -- they allegedly killed US journalist James Foley and Western aid workers.

The cell also included Mohammed Emwazi -- known as "Jihadi John" -- who was killed in an air strike in 2015, and Aine Davis, who has been jailed in Turkey.