While Trump did not give any specifics, the Washington Post reported that US forces had gained control of two notorious British fighters dubbed "The Beatles" and moved them to Iraq.
CNN also reported that the US military had taken custody of the two fighters, while ABC said they had been transferred to a safe location.
"We are taking some of the most dangerous ISIS fighters out and we're putting them in different locations where it's secure," Trump said, using an acronym for the Islamic State group.
"We have taken a certain number of ISIS fighters who are particularly bad and we've wanted to make sure nothing happened to them with respect to getting out," he said.
The move addressed one of the most worrisome issues of Trump's green light to Turkey to invade Syria, where the Kurds, a longtime US battlefield partner, are viewed as a terror threat by Ankara, a NATO ally of Washington.
The Kurdish-led Syrian Democratic Forces have been holding prisoner some 10,000 captured Islamic State group fighters.
Many have feared the Turkish attack would lead to them escaping and reconstituting the group.
The SDF-held fighters include around 2,000 of foreign nationality, many of them from European countries that have refused to take them back.
The two "Beatles" reportedly in US custody, Alexanda Kotey and El Shafee Elsheikh, were part of an extremely violent all-British four-man cell that kidnapped and tortured foreigners, including journalists, at the height of Islamic State group's power in Syria and Iraq.
One other was killed in a drone strike and the fourth is imprisoned on terror charges in Turkey.
Trump said the Kurds are still guarding many of the Islamic State group militants, but also said Turkey will be responsible for them.
"If the Kurds don't watch, Turkey will watch. They don't want those people out any more than we do," he said.
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