Fears are growing that the Foreign Office has paved the way for British Islamic State captives held in Syria to be handed over to the regime of President Bashar al-Assad, which is accused of war crimes including mass torture and executions.

Human rights groups argue that, as they face pressure to repatriate Britons detainedin Syria, ministers’ inaction may result in the transfer of UK nationals to Assad’s forces.

The legal charity Reprieve says that differing statements from the Foreign Office indicate that the government has diluted its previous opposition to British nationals being prosecuted in Assad-controlled Syria.

In April, parliament was told by the then Foreign Office minister Mark Field that, “given the regime’s appalling human rights record, we would not view prosecution by the Assad regime as an appropriate means of justice”.

However, the Foreign Office recently appeared to offer a changed stance when asked what steps were being taken to stop Britons being transferred to Assad forces. Andrew Murrison, the current minister of state with responsibility for the Middle East , said the UK would work not only with international agencies but also “partners in the region who can assist in establishing an appropriate pathway to justice within their territory or legal systems”.

Last month Assad, who has slowly regained control over most of Syria after a nine-year war that claimed at least 500,000 lives, said foreign nationals detained in Kurdish-held areas would be tried in his regime’s “specialised courts” as his forces move into the north-east.

About 10,000 Isis fighters are being detained by western-backed Kurdish forces in north-east Syria, including 2,000 foreigners and about 30 British men. Thousands more foreign women and their children are also being held there, including scores of UK nationals and at least 60 children.

Maya Foa, the director of Reprieve, said it had written to the foreign secretary, Dominic Raab, asking him to clarify the UK’s position and what the government believes is “an appropriate pathway to justice”.

“Sending Brits to Assad’s Syria would be effectively sentencing them to torture, disappearance or summary execution – abuses that are directly opposed to fundamental British values and the law,” said Foa. “Yet this government appears unwilling to categorically rule this out as an option. They must do so at once, and begin efforts to repatriate British detainees to the UK where children may be given the support they require and adults may, if appropriate, face justice in British courts.”

Sending Brits to Assad's Syria would be effectively sentencing them to torture, disappearances or summary execution Maya Foa, Reprieve

The UK government’s most recent human rights and democracy report acknowledges reports of “widespread and systematic use of arbitrary detention, torture and execution of detainees” in Syrian prisons.

A Human Rights Watch investigation in 2015 revealed at least 6,700 killings in Syrian detention. Last May human rights lawyers filed war crimes cases against Assad at the International Criminal Court in The Hague.

The Foreign Office said the transfer and prosecution of Isis detainees was “ultimately a matter for authorities under whose jurisdiction the individuals are detained”. They should “face justice through a fair trial in the most appropriate jurisdiction”.

It was concerned that Assad-held parts of Syria authorised the death penalty. “The UK has significant human rights concerns about trials of Daesh-related suspects in domestic courts of Iraq and regime-held areas of Syria including the use of the death penalty, which the UK opposes in all circumstances, as a matter of principle,” said the FCO.

Although the government has repatriated a few British orphans, it has failed to act over the majority of British children and adults in camps over which the Kurdish authorities say their control is declining.

“To ensure the UK does not lose track of British detainees in north-east Syria as a result of their transfer to Assad-controlled forces, the government must act now to repatriate all UK nationals held by Kurdish authorities,” said Foa.