Britain and other European nations that are refusing to repatriate Islamic State fighters and put them on trial in their country of origin are creating a risk to regional security, the US defence secretary warned at the start of his visit to London.

Mark Esper said there were around 2,000 foreign fighters, many from Europe, held in north-east Syria, but asking the Kurdish-led Syrian Democratic Forces to keep them in makeshift jails was an increasing risk to the fragile security of the region.

“It’s an untenable situation,” Esper said in a briefing on the first day of a two-day visit to London. “How long can this last? Our view has been they should repatriated and dealt with appropriately … otherwise that’s a risk to the region.”

Around 250 to 300 foreign fighters who are still in Syria are estimated to have come from the UK, but Britain is increasingly unwilling to allow any to return and stand trial following Isis’s defeat in Syria and Iraq.

Several have been stripped of their British citizenship, such as Jack Letts, who was raised in Oxfordshire by British and Canadian parents. He left home to join Isis five years ago, but has been held a prisoner in Syria for the past two years.

Esper, who is due to meet defence secretary Ben Wallace on Friday, said he was relaying a message from the US administration. “We are asking a lot of the folks that are holding them, the Kurds,” Esper added.

Two of the most notorious Isis fighters still being held in north East Syria, Londoners El Shafee Elsheikh and Alexanda Kotey, are subject of an ongoing legal battle as to where they will be put on trial for murdering western hostages in Syria.

Wallace, in his previous role as security minister, said in 2018 that it was not possible to try them under UK law, and that they must instead be sent to the US. However, Washington is refusing to assure the UK that they would not face the death penalty there.

Elsheikh’s mother has brought a case to the supreme court asking that they be put on trial in the UK. Her lawyers argued in front of Britain’s highest court that there was enough evidence to bring them to the UK for trial.

The subject is a running sore between the United States, the UK and other European countries. Last month, president Donald Trump even bizarrely threatened to release Isis fighters “into the countries from which they came. Which is Germany and France and other places.”

Concerns have also been raised by the US that the conditions in which foreign fighters are held could radicalise them further, with some officials raising the prospect there could be a repeat of what happened in Iraq a decade ago.

Many of the men who ended up leading Isis, including its leader, Abu Bakr al-Baghdadi, were detained at Camp Bucca, a US detention facility in Iraq, and are thought to have met there.

A total of 850 Britons are estimated to have travelled to join Isis in Syria and Iraq, joining the self-styled caliphate known for the brutal killing of hostages. A third of the combatants are estimated to have been killed, while another third are thought to have independently made it back home in the earlier stages of the conflict, where they remain under some degree of monitoring.

Esper is due to give a speech in London on Friday where he is expected to warn that “the rules-based order is increasingly under challenge from Russia and China”.

When asked if the UK needed to be more assertive on China, Esper said: “We all need to be more robust on China.” He said he was in Hong Kong at the time of the 1997 handover from the UK to China, and added: “Look at what is happening today. Does that look like they are living up to their agreements?”