(CNN) The US military has taken two high-profile members of a British ISIS cell into custody, according to three US officials. Two officials said the transfer was made Wednesday.

It comes amid US concerns that the Turkish offensive in northeastern Syria could result in ISIS prisoners escaping from undermanned prisons guarded by Syrian Democratic Forces inside Syria.

President Donald Trump tweeted early Thursday morning that 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 U.S. They are the worst of the worst!"

One of the US officials said there are plans to bring the two ISIS members, Alexanda Kotey and El Shafee Elsheikh, to the US for prosecution. The two, who have been held in northern Syria by the US-backed Syrian Democratic Forces for more than a year, were part of an ISIS cell dubbed the "Beatles."

The Washington Post was first to report t he two men have been taken into US custody.

Officials would not say where the men are currently being held by the US military but one official said they had not yet been moved to the US. A US defense official confirmed that "two high-profile ISIS prisoners" had been transferred from Syrian Democratic Forces detention to US military custody Wednesday but would not confirm their identities. The official said they were now being held in a secure location outside Syria in a manner "pursuant to the law of armed conflict."

Kotey is accused by the US State Department of having "likely engaged in the group's executions and exceptionally cruel torture" of Western journalists and aid worker hostages. Elsheikh "was said to have earned a reputation for water-boarding, mock executions, and crucifixions," according to the State Department.

Their ISIS execution cell is accused by the State Department of "holding captive and beheading approximately two dozen hostages," including James Foley, American journalist Steven Sotloff and American aid worker Peter Kassig.

The US effort to take them into custody has moved in fits and starts in recent months. Complications arose because of British legal issues that could prevent the UK from sharing evidence the US needs to prosecute the men.

Given the fast-moving developments in Syria, Attorney General William Barr in recent days asked President Donald Trump to make the transfer a priority, and he signed off.

US officials have long warned of the vulnerability of the "pop-up prisons" housing some 11,000 to 12,000 ISIS fighters captured on the battlefield, 2,000 of whom are foreigners not from Iraq or Syria. Several hundred of the prisoners are believed to be from Europe.

CNN interviewed the pair in July

In an interview with CNN in July, Kotey and Elsheikh confessed their part in the ransoming of Western hostages. Elsheikh also offered an unprecedented apology for his actions with the group.

A visibly drained Elsheikh said: "I consider my role in this whole scenario, this whole episode, as one of my mistakes that I would like to apologize for. [To] everybody involved and everybody who was affected, directly or indirectly."

Kotey declined to offer an apology. But he admitted taking email addresses from European hostages and assisting in the ransom negotiations that followed with relatives and friends.

"I was a fighter," Kotey said. "Extracting from them email addresses for communications. For example, if it was a proof of life question, something that only they would be able to answer." Asked why he agreed to this task, he replied: "It just so happened that way."

Elsheikh said he was involved in the same activities, "initially just liaising between the foreigner prisoners and the people dealing with their negotiations process." He confirmed that this had involved negotiating ransoms.

The men denied involvement in the murders and physical abuse of hostages, saying they had been transferred to another unit before the violence began. Several former hostages, however, have said they were tortured by masked British-accented men matching the men's descriptions.

Ricardo Garcia Vilanova, a Spanish photographer imprisoned by the group for about six months before his release in 2014, told CNN: "I was tortured in an ISIS jail by masked men with British accents. Many other of my fellow prisoners were too, and some didn't survive captivity. We would want that they see justice for what they did."

The United Kingdom stripped the men of citizenship as far back as 2015, and it now refuses to put them on trial, citing the legal complications of repatriating former citizens.

Elsheikh said: "If anything, I think that a confession will maybe hasten our extradition or rendition to the United States. I don't think this is something that will prevent me going to the United States at all. I don't see how that would be possible. I just want this period to be over. I know what needs to be done. The truth has to come out."

Kotey admitted that he had helped organize a failed London-based ISIS assassination plot in 2016 remotely from Syria. He claimed not to have known the target and then later learned it had been a Kuwaiti Shia cleric living in London in exile. The plot was actually intended to kill British soldiers or police, a UK trial later heard.

"I was responsible for his acquisition of a firearm," Kotey said. "As far as the details of any plot ... I had no involvement in that."

Both men expressed their preference to be tried in a British court, a possibility that seems remote now they are in US custody.

The US government has long sought to encourage countries to repatriate their citizens, and the Department of Justice has been able to charge several American citizens for their alleged involvement with ISIS.

However, the effort to get other countries to repatriate their citizens from Syrian Democratic Forces detention camps has had limited success to date, with only a handful having publicly repatriated their fighters.

Many countries are reluctant to repatriate because of the difficulty of prosecuting suspected ISIS members based on evidence collected on the battlefield.

The US has to date largely resisted taking custody of non-US citizens who fought for ISIS.