WASHINGTON (Reuters) - The British government is asking U.S. authorities to promise they will not seek the death penalty if two Islamic State militants detained in Syria are extradited to the United States, according to European government sources.

The British also want the U.S. to guarantee that if the two men, members of a group of British Islamic State recruits, are extradited, they will not be sent to the military prison at Guantanamo Bay, Cuba.

Instead, British officials are seeking assurances that they will be tried in civilian courts, the sources said, speaking on the condition of anonymity. The British government’s position was first reported by the New York Times.

The two, Alexanda Kotey and El Shafee Elsheikh, nicknamed “The Beatles” for their hairstyles, were captured by a Kurdish militia unit in Syria that is still holding them. They and other British extremists, including a former Londoner known as Jihadi John, belonged to a team that executed Americans taken hostage by the group.

Jihadi John, whose real name was Mohammed Emwazi, was killed by a U.S. drone strike in November 2015.

British officials have said the UK does not want the men repatriated to the United Kingdom, which withdrew their British citizenship.

One of the sources said Britain opposes the death penalty, but added that where there is evidence they have committed a crime, “Foreign fighters should be brought to justice regardless of their nationality. The appropriate process will be dependent on the individual circumstances.”

The other source said Britain will “always press the U.S. not to apply the death penalty on security cases where we collaborate.”

The U.S. Justice Department had no comment.

Legal experts said that in the past, the U.S. has offered foreign governments the kind of assurances Britain is seeking.

Joshua Dratel, a New York lawyer who has defended alleged extremists in U.S. courts, said, “It is ordinarily a condition of extradition: no death penalty, no Guantanamo.”

“Without assurances that the U.S. won’t cause our allies to violate their human rights obligations, cooperation would, and indeed should, dry up,” said Hina Shamsi of the American Civil Liberties Union.