Mark Esper has been US Defense Secretary since July 2019 (Picture: PA).
The US Defense Secretary says the UK and other European countries must take back its so-called Islamic State (IS) fighters back so they can "face justice".
Mark Esper, who is meeting the British Defence Secretary in London later, warned the number of foreign fighters in Kurdish jails in Syria is an "untenable situation".
“Our view is they should be repatriated and dealt with appropriately, so that’s the message,” Mr Esper said.
“Otherwise it’s a risk to the region. How long can they be guarded in these camps by others?
"You’re talking about several thousand fighters, but there are a little over 2,000 foreign fighters, many of which come from Europe."
Around 200 to 300 foreign fighters who are still in Syria are believed to have come from the UK, according to reports.
Mr Esper also criticised the stripping of citizenship as a way of dealing with the problem - something the UK has recently done.
Just a few weeks ago, Jack Letts, nicknamed 'Jihadi Jack', had his British citizenship removed.
Mr Esper said the stripping of citizenship "doesn't resolve the fact" the fighters are still being held in Kurdish prisons.
He went on to say they "should be repatriated" and "dealt with by their countries of origin".
"I think they should all go back to their home countries to face justice," Mr Esper continued.
Mr Esper, who is visiting London for the first time since being confirmed as US Defense Secretary, will call on the UK to follow the US' stance.
The two countries are seemingly at odds over what to do with foreign fighters and their families caught on the battlefield in Syria.
The UK Defence Secretary Ben Wallace has previously backed the policy of stripping some foreign fighters of their British citizenship.
Mr Esper is also expected to use his visit to raise concerns about China and the role of its telecom company, Huawei.