Washington (AFP) - Two notorious Islamic State jihadists dubbed "The Beatles" who were held by Syrian Kurds are now in US custody and have been moved out of the country, President Donald Trump said early Thursday.

Turkey has launched an assault on the Syrian Kurdish forces -- with which the US partnered to combat Islamic State militants -- sparking fears that the offensive could lead to captured fighters they held escaping and reconstituting the group.

"In case the Kurds or Turkey lose control, the United States has already taken the 2 ISIS militants tied to beheadings in Syria, known as the Beetles, out of that country and into a secure location controlled by the US," Trump tweeted.

"They are the worst of the worst!"

The pair were part of an extremely violent all-British four-man cell that kidnapped and tortured foreigners, including journalists, at the height of the Islamic State group's power in Syria and Iraq.

A US defense official had earlier confirmed they had taken custody of two "high-value" IS individuals from the Kurdish-led Syrian Democratic Forces (SDF) that held the captured jihadists.

"They have been moved out of Syria and are in a secure location," the official said, without identifying where. "They are being held in military custody pursuant to the law of war."

One other member of the four-man jihadist cell was killed in a drone strike and the fourth is imprisoned on terror charges in Turkey.

- 'Particularly bad' -

Captured British Islamic State (IS) fighters El Shafee el-Sheikh (L) and Alexanda Kotey (R), two members of the so-called 'Beatles' kidnapping and torture cell (AFP Photo/Handout)

Their cell is accused of abducting and decapitating around 20 hostages including American journalist James Foley, who was beheaded in 2014.

Trump had earlier said the US was taking steps to prevent the potential escape of particularly dangerous Islamic State group fighters amid the Turkish offensive.

"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 at the White House.

"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.

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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 SDF have been holding prisoner some 10,000 captured Islamic State group fighters.

The SDF-held fighters include around 2,000 of foreign nationality, many of them from European countries that have refused to take them back.

Trump said the Kurds were still guarding many of the Islamic State group militants, but also said Turkey would 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.