Two British jihadists, believed to be part of an Islamic State (ISIS) group that beheaded hostages and was known as “The Beatles”, have been moved out of a detention center in Syria and are in American custody, US officials said Wednesday, according to a report in The Associated Press.

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

The two men, El Shafee Elsheikh and Alexanda Amon Kotey, along with two other British jihadists, allegedly made up the ISIS cell nicknamed “The Beatles” by surviving captives because of their English accents.

The group's leader, Mohammed Emwazi who was known as “Jihadi John, was killed in an airstrike in 2015 in Syria after an intensive manhunt. “Jihadi John” was seen in ISIS videos showing the beheadings of journalists James Foley and Steven Sotloff, British aid workers David Haines and Allan Henning, and Japanese hostage Kenji Goto.

A fourth member of the so-called “Beatles”, Aine Davis, is imprisoned in Turkey on terrorism charges.

All four had lived in West London. Kotey, born in London, is of Ghanaian and Greek Cypriot background, while Elsheikh’s family fled Sudan in the 1990s. Both men have been designated foreign terrorists by the United States.

US officials said on Wednesday the two were taken out of Syria by US 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 ISIS jihadists removed from Syria by the US, but officials say a number of others could also be moved if needed.

The officials, who spoke on condition of anonymity to discuss military movements, would not say how many could be relocated or where the two British men were taken.

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

In March, Kurdish and US 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.

Reports of them being moved came after Turkish President Recep Tayyip Erdogan confirmed earlier on Wednesday that Turkish military activity in Syria has begun.

The Turkish operation followed the White House's announcement on Sunday that the US will pull back its forces from parts of northern Syria.