Two British Isis members accused of involvement in the beheading of western hostages held in north east Syria have been taken into US custody as a Turkish offensive on the Kurdish region begins.

Alexanda Kotey and El Shafee Elsheikh, were members of a British group of Isis militants known as “the Beatles” and are understood to be amongst a number of fighters in the process of being transferred to the Americans.

US officials briefed media on the custody operation overnight, with the action first reported by the Washington Post. Prisoners previously held by the Syrian Kurds were first expected to be taken to neighbouring Iraq.

Donald Trump said: “We have taken a certain number of Isis fighters that are particularly bad, and we’ve wanted to make sure that nothing happened with them with respect to getting out.”

The US president subsequently tweeted: “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 U.S. They are the worst of the worst!”

Trump said he had spoken to Boris Johnson on the subject of Isis prisoners, but did not say whether he was referring to Kotey and Elsheikh. It is not clear how many prisoners are involved in the transfer.

The duo were part of a group of four who are accused of being involved in the apparently filmed beheadings of British aid workers David Haines and Alan Henning and American journalists James Foley and Steven Sotloff.

Group leader Mohammed Emwazi was killed in a US drone strike in 2015, while another Aine Davis was caught in Turkey and jailed for seven and half years in 2017, for being member of a terror organisation.

Estimates vary about the number of foreign Isis fighters held by the Kurdish military, the Syrian Democratic Forces, the SDF, but their numbers are at least 1,000 and potentially double that. There are also an estimated 10,000 or so Isis fighters in detention from Syria and Iraq.

The number of Britons held in Isis jails is estimated by security source to be around 30, and the UK has largely pursued a policy of ignoring them, arguing that they travelled to Syria at their own risk, a country where there has been no consular support since the start of the civil war in 2011.

One of those held is Jack Letts, who was raised in Oxfordshire and fled to join Isis before he was picked up in 2017. His British citizenship was stripped by the UK government over the summer, leaving him with his Canadian nationality inherited from his father. His situation is unclear.

There had been repeated warnings that the Turkish invasion meant that the Isis fighters could end up being released by one of the parties to the looming conflict, but Trump’s words suggest that the US intended to mitigate some of that risk.

British officials said they could not confirm what was happening, although there indications that the UK had been briefed in advance on the plans. A UK Home Office spokesperson said on Thursday: “It would be inappropriate to comment whilst legal proceedings are ongoing.”

Earlier this month, a cross-party group of MPs and peers who visited the region last month warned that Trump’s green light to a Turkish invasion “risks global security” because it could allow Isis members to escape and regroup.

The two British Isis fighters had been at the centre of a legal battle in London, led by Elsheikh’s mother who went to the Supreme Court in London to stop his extradition to the US, and to prevent the British government sharing evidence with the US, if the death penalty is not ruled out. Judgment in that case is awaited.

Meanwhile, women and children in the largest Islamic State detention centre in Kurdish-controlled Syria are expecting to be freed in the wake of a Turkish assault on the area, according to people inside the camp.

Al-Hawl, home to about 60,000 women and children with links to Isis and 10,000 displaced civilians, has been tense since Donald Trump announced US troops would leave the area at the weekend, paving the way for the Turkish attack on Wednesday.

Radicalised women, including some who have been accused of killing other prisoners they say are not adhering to Isis’s strict ideology, believe Isis sleeper cells in the area will attack the Kurdish guards and free those inside in the next two days, a woman who has been held alongside them said via WhatsApp message.

“They know the Turkish campaign has begun,” the woman said. “After living in this horrible place for months they are ready to take this opportunity to break out.”

About 90,000 men, women and children from Isis’ former “caliphate” are currently in the custody of the US-backed Kurdish-led Syrian Democratic Forces (SDF).

The US president had shifted his position repeatedly on the detainees, since Sunday night when he effectively gave the Turks the green light to invade the Kurdish controlled part of Syria, suggesting initially that Turkey might have to take responsibility for them.

But it is unclear what would happen to the other Isis detainees not covered by the US operation. Last night, Trump railed against European countries for not taking custody of their own nationals who had been caught in Isis ranks – and suggested they could escape.

“Well, they’re going to be escaping to Europe. That’s where they want to go. They want to go back to their homes, but Europe didn’t want them from us,” the president said.

British and European officials say they fear that trials in home countries could prove difficult because the offences took place overseas, in Syria and Iraq, and the witnesses and evidence are in those countries.