WASHINGTON -- Two British militants believed to be part of an Islamic State group that beheaded hostages and was known as "The Beatles" have been moved out of a detention centre in Syria and are in American custody, U.S. officials said Wednesday.

President Donald Trump said earlier Wednesday that the U.S. has moved some of the Islamic State prisoners amid fears some could escape custody as Turkey invades northeast Syria.

The two men, El Shafee Elsheikh and Alexanda Amon Kotey, along with other British jihadis, allegedly made up the IS cell nicknamed "The Beatles" by surviving captives because of their English accents. In 2014 and 2015, the militants held more than 20 Western hostages in Syria and tortured many of them. It beheaded seven American, British and Japanese journalists and aid workers and a group of Syrian soldiers, boasting of the butchery in videos released to the world.

U.S. officials said the two were taken out of Syria by U.S. military and law enforcement personnel to ensure they did not escape if security broke down as a result of the Turkish incursion. So far they are the only two IS militants removed from Syria by the U.S., but officials say a number of others could also be moved if needed.

Another official said the two men were taken to Iraq out of an abundance of caution, adding that the United States is still fully committed to seeing them brought to justice.

The officials spoke on condition of anonymity to discuss military movements.

Turkey is attacking the U.S.-backed Syrian Defence Forces, a Kurdish force that battled the Islamic State group alongside American troops and now is responsible for guarding thousands of detained militants.

But guarding those prisoners is now expected to be less of a priority for the Kurdish forces as they rush to defend their territory against the invading Turkish military.

Trump told reporters at the White House that some of the "most dangerous" had been moved, but he provided no details. Although just the two have been relocated so far, but thousands of other Islamic State group detainees remain in custody and there are no immediate concerns the Kurds will completely abandon the facilities.

"We're putting them in different locations where it's secure," Trump said.

The U.S. officials said that some Kurds left the prisons to join the fight but did not flee in large numbers. The officials said operations against remaining members of the Islamic State group are on hold following the invasion Wednesday by Turkey, which sees the Kurds as a threat and is trying to create a buffer zone between the territory held by the SDF and the Syrian border.

The two British men were captured in January in eastern Syria by the Kurdish forces amid the collapse of IS. Their detention set off a debate in the U.S. and Europe over how to prosecute their citizens who joined IS.

Among the journalists they killed was American James Foley, who was first, followed by fellow Americans Steven Sotloff and Peter Kassig, British aid workers David Haines and Alan Henning and Japanese journalists Haruna Yukawa and Kenji Goto. The beheadings, often carried out on camera, horrified the world soon after IS took over much of Iraq and Syria in 2014.

In March, Kurdish and U.S. forces cleared the last members of the Islamic State group from what was left of their self-declared caliphate, which once sprawled across a large part of Iraq and Syria.

U.S. officials said American forces are not out doing patrols looking for Islamic State group fighters because their Kurdish partners are more focused on the Turkish fight. But they said U.S. operations could restart quickly if needed.

About 30 to 50 U.S. troops were moved out of the way from two outposts in the border region. There are a number of U.S. forces in other bases just outside the so-called safe zone as well as in Manbij and other locations around the country. They have not been moved but are mainly staying in place to avoid attacks.

There are about 2,500 Islamic State foreign fighters being detained in Syria, along with about 10,000 fighters from Syria and Iraq.

Trump and other U.S. officials have repeatedly pressed other nations across Europe and the Middle East to take back the detainees from their countries. But international leaders have been largely reluctant and have been slow to take any back.

"They should go back, by the way, they should go back to Europe. Many of them came from Europe. And they should go back to Germany and France," Trump said Wednesday.

Trump said other leaders told him they didn't want the detainees.

"We don't want them either," he said. "Nobody wants them but they're bad and somebody has to watch over 'em."

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Associated Press writers Sarah El Deeb in Beirut and Eric Tucker in Washington contributed to this report.