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

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 U.S.-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. "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 U.S. 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-N.H., said in a statement.

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.

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 U.S. officials.