Kurdish-led authorities in north Syria said they would not free foreign detainees of the so-called Islamic State, but countries must take responsibility for them, after US President Donald Trump said jihadists would be freed unless Europe takes them back.
Abdulkarim Omar, co-chair of foreign relations in the Kurdish-led region, said around 800 foreign fighters were being held in prisons, along with around 700 wives and 1,500 children in camps for the displaced.
Dozens more detainees and relatives were arriving by the day.
He described the detainees as a "time bomb", saying fighters could escape if the Kurdish-led autonomous area were attacked.
British Prime Minister Theresa May today rejected Mr Trump's call and said prosecutions should usually take place in the country the crimes were committed.
Mr Trump used Twitter posts to call for the UK, France and Germany to take back captured fighters from IS - also known as ISIS - and warned if they did not the US would be forced to release them.
"The US does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go," Mr Trump said.
"We do so much, and spend so much - Time for others to step up and do the job that they are so capable of doing."
Mrs May's official spokesman said: "Foreign fighters should be brought to justice in accordance with due legal process in the most appropriate jurisdiction.
"Where possible, this should be in the region where the crimes had been committed.
"We continue to work closely with our international partners on this. The government will do all it can to ensure the safety and security of the UK."
Additional Reporting PA