El Shafee Elsheikh, one of detained accused of involvement in the ISIS executions of Western hostages.

Two British men accused of involvement in the ISIS's summary executions of Western hostages, including Americans, are being transferred into US military custody because the Turkish incursion into Syria threatened their continued detention by Kurdish forces in northeastern Syria, according to US officials.

The pair - part of a group of four British militants dubbed the "Beatles" by their hostages - were being detained with the goal of putting them on trial in the United States, said a senior US official, who like others spoke on the condition of anonymity to discuss a sensitive matter. That official said the two men had been taken to Iraq; other officials said they and other high-value detainees were being placed in US military custody but could not say where they were being taken.

A criminal prosecution in the United States rests on the ability to obtain evidence from British authorities - a matter being litigated in the Supreme Court of the United Kingdom, the official said.

The British men are accused of involvement in the beheading of journalist James Foley and other Western hostages.

The disclosures come in the wake of President Donald Trump's decision announced late Sunday to begin pulling troops out of northeastern Syria, effectively greenlighting the Turkish offensive that began Wednesday against US-allied Kurds.

The Turkish attack on Kurdish forces raised concerns about the ability of the Kurds to maintain control over thousands of ISIS detainees, as well as tens of thousands of women and children housed in separate camps, some of whom are militant supporters.

"This is like a victory for the ISIS fighters. I just think it's appalling," said Diane Foley, the victim's mother, using an acronym for the ISIS. "It's an abdication of our responsibility to ensure safety for our own citizens and allies."

If the Kurds abandon detention facilities to press all of their troops into the fight with Turkey, officials say the US Defense Department has orders not to intervene beyond moving the group of high-value detainees to Iraq. Kurdish officials have warned they are already struggling to control a camp that holds as many as 30,000 ISIS loyalists, including about 10,000 foreigners.

"We now face the very real prospect of 10,000 ISIS prisoners rejoining the battlefield," Sen. Jeanne Shaheen, D-NH, said in a statement Wednesday.

Mohammed Emwazi, the man who killed Americans James Foley, Steven Sotloff, Peter Kassig and other hostages in 2014, was killed in a drone strike the following year. A fourth American, Kayla Mueller, was killed while being held hostage by ISIS, but the exact cause of her death was not confirmed.

Two of Emwazi's British associates, Alexanda Kotey and El Shafee Elsheikh, had been in custody of the Kurdish-led Syrian Democratic Forces.

Their potential transfer to the United States has been delayed by Elsheikh's mother, Maha Elgizouli, who has challenged the British government's decision not to prosecute her son in Britain. She has also sued the British government to block any evidence-sharing with U.S. prosecutors without legal assurance that her son will not be executed.

"Mrs. Elgizouli is solely concerned to protect her son from the death penalty," attorney Edward Fitzgerald said in a July hearing before the Supreme Court of the United Kingdom. "She recognizes that they should face justice. . . . But she submits that they should face justice in this country."

Prosecutors in the United States would seek to convict Kotey and Elsheikh as conspirators in hostage-taking resulting in death, a charge that carries a potential death sentence, according to US officials.

In an interview this summer, Kotey and Elsheikh denied involvement in any murders, saying they only facilitated ransom negotiations. Both men agreed to speak to The Washington Post, and Kurdish security officials facilitated separate interviews at a facility in Rmeilan, Syria.

Their role, both said, was to ask prisoners for contact information and personal details for "proof of life." Kotey recalled having prisoners hold up signs urging their governments and families to "be quick or they will be kill me."

At one point, Kotey said, a Syrian prisoner was shot in the back of the head in front of the European prisoners, who were made to hold signs saying they wanted to avoid a similar fate.

The British and American hostages were not included in that video, he said, because their governments were not negotiating.

"They were not pampered," Elsheikh said. "The treatment had to be harsh to keep them in the state of mind" of compliance. "The prisoners had to be kept always under pressure."

He said the harsh treatment included headlocks, punches and stress positions. But he denied any involvement in mock executions or waterboarding.

Kotey said he saw Emwazi, better known as "Jihadi John," beat prisoners and threaten to waterboard them "as if he had previously." He said Emwazi saw the killing of journalists and aid workers as warranted because they had "come to interfere in our internal affairs."

Both Kotey and Elsheikh claim they were no longer working with Emwazi when the beheadings began but earlier were involved in the detention of Western hostages. But they say they were among a very small group of ISIS members who knew Emwazi's true identity, first reported in The Washington Post in early 2015.

A decision is expected in the coming weeks from the Supreme Court of the United Kingdom on whether the British government's offer to share evidence on Elsheikh and Kotey, absent a promise from the United States that the men will not face the death penalty, violates British law.

Elsheikh said he and Kotey were captured by the Syrian Democratic Forces trying to cross into Turkey.

Toby Cadman, a British lawyer representing Diane Foley, said he also worries that moving the prisoners around could create new opportunities for the defendants' families to delay a prosecution.

"The last thing anyone wants is for the process to be . . . fudged in order to get them before a court that they can then challenge," he said. "You want these people lawfully handed over."

An attorney for Elsheikh's mother has said that British prosecutors could charge Kotey with murder and Elsheikh with at least membership in a terrorist organization. The latter crime carries a maximum of 10 years in prison. But British authorities have for years said they would prefer to see the two charged in the United States.

A fourth "Beatle," Aine Davis, was convicted in Turkey of membership in a terrorist organization and sentenced to seven years in prison.

Foley recalled a 2014 event at which Trump praised her son as "brave and noble" and told her how important accountability for his death was.

"I would implore the president to remember the four Americans who were brutally tortured and killed," she said. "I am hoping that our country has a plan."