Two British ISIS fighters known as “The Beatles” have been moved out of their Syrian prison and are in US custody.
The prisoners were transferred along with others amid fears some could escape custody as Turkey invades northeast Syria, officials said.
The two men, El Shafee Elsheikh and Alexanda Amon Kotey made up part of the ISIS cell nicknamed The Beatles.
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.
US officials said 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.
Turkey is attacking the US-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.
US President Donald Trump told reporters at the White House that some of the "most dangerous" had been moved.
"We're putting them in different locations where it's secure," Mr Trump said.
Agencies contributed to this report