The United States will not pursue the death penalty against two members of a notorious Islamic State kidnapping and murder unit dubbed the "Beatles," Attorney General Bill Barr said in a letter released by the Justice Department Wednesday.

In a breakthrough in a two-year impasse between Washington and London, Barr told British Home Secretary Priti Patel that Alexanda Kotey and El Shafee Elsheikh, who allegedly took part in grisly videotaped beheadings of Westerners, would be spared execution if placed on trial in the US.

Barr made the pledge to be able to secure crucial evidence against the two Britons, now stripped of their citizenship, which has been withheld by the British government due to the chance they would face capital punishment, which is outlawed in Britain.

"If imposed, the death penalty will not be carried out," Barr said in the letter dated Tuesday.

The letter put the onus on Patel to see if the British system can overcome a tough court challenge by Elsheikh's mother and share the evidence, said to be damning wiretaps, with US authorities.

The announcement came on the sixth anniversary of the killing by IS of James Foley, a journalist who worked for AFP and other news organisations.

"This is a major breakthrough," his mother Diane Foley said, describing Barr's decision as "a huge step which they were not willing to do earlier."

Grisly beheadings

The United States wants to try the two for the murder of journalists Foley and Steven Sotloff and aid worker Peter Kassig during a period in 2014-2015 when IS used videos of their and others' deaths as propaganda.

The four-member unit Kotey and Elsheikh belonged to was dubbed "the Beatles" by their captives due to their British accents.

They tortured and killed victims, including by beheading, and IS released videos of the deaths for propaganda purposes.

Kotey and Elsheikh are being held by US forces in Iraq.