British Islamic State fighters held in north-east Syria could end up being released if the US allows Turkey to invade the territory, UK politicians and the parent of one detainee have warned.

A cross-party group of MPs and peers who visited the region last month warned that Donald Trump’s unexpected Sunday night decision to give Turkey a free hand “risks global security” because it could allow Isis members to escape and regroup.

US withdrawal from Syria leaves fate of Isis fighters and families in detention uncertain Read more

Chaired by Labour backbencher Lloyd Russell-Moyle, the all-party group on north-east Syria said it was unclear what would happen to the estimated 18,000 Isis fighters and 70,000-plus refugees, whose numbers include just over 60 adult Britons.

The group questioned whether “these dangerous people will remain detained and not be released to find themselves in Athens or London or be used as a bargaining chips against British interests.”

Amongst those held are two of the so-called Beatles, accused of participating in notorious beheadings of hostages, Britons Alexanda Kotey and El Shafee Elsheikh, as well Shamima Begum and Jack Letts, both of whom had have their British citizenship stripped in the past year.

The prisoners are currently held by the Syrian Democratic Forces (SDF), a 75,000-strong force that provided the majority of the ground troops in the military campaign to eliminate Isis territorially in Syria, a struggle that ended in March when Isis was expelled from Baghuz.

Britain and other European nations have largely refused to repatriate fighters and put them on trial in their own countries to the frustration of the United States. The Kurdish-dominated administration has alternatively suggested creating a local war crime tribunal.

Other politicians in the group are Conservative MPs Crispin Blunt, a former chair of the foreign affairs select committee, and Adam Holloway plus Lord Glasman, a Labour peer, and former adviser to the party’s previous leader Ed Miliband.

Members of the group also expressed anxiety that the Turks might begin bombing raids using F16 jets, previously deployed against separatist Kurds in the south of their own country, in a region where aerial superiority provides a critical military advantage.

The politicians’ warning was echoed by Sally Lane, the mother of Letts. She said she believed that “the Kurds could suddenly release them” or that “they’ll ask people if they want to fight for them against Turkey” to deal with the sudden threat.

Lane wants her son to be extradited to Canada and face justice there after the UK stripped him of his citizenship over the summer. A Muslim convert, Letts quit his Oxfordshire home and joined Isis before being picked up by the SDF in 2017 where he has been in a crowded prison ever since.

She said that she was “terrified, what’s going to happen to Jack. I’m worried that he could get killed in the crossfire”, adding: “I know what people think, who cares about hundreds of Isis fighters, but one of them is my son.”

The White House said in its Sunday night statement that it expected Turkey to take responsibility for the Isis prisoners and others if it did mount military operations, prompting additional questions.

İbrahim Doğuş, the founder of the London-based Centre for Kurdish Progress, said if an invasion were to take place “the SDF may not be able to control the camps and prisons due to relocating its forces while Turkey may decide to release them to use against SDF and then let them go anywhere they want”. Both scenarios were dangerous, he said.

The British government attempted to sound a note of concern on Monday morning in the immediate aftermath of the unexpected White House announcement. A Downing Street spokesman said the priority for the UK was to secure the lasting defeat of Daesh [Isis] and added: “We would be concerned by any unilateral action that would threaten the progress we have made towards this aim.”

Kurdish politicians have repeatedly said that they need help in dealing with Isis prisoners and in the refugee camps, where support for the Islamist group remains strong. Last month Isis’s leader, Abu Bakr al-Baghdadi, released an audio message calling on Isis fighters to liberate prisoners and those living in refugee camps.