Donald Trump has called on Britain and other European allies to take back more than 800 Islamic State fighters captured in Syria and put them on trial.

The US president said "the alternative is not a good one in that we will be forced to release them", with the terror group's caliphate in Iraq and Syria "ready to fall".

He tweeted: "The US does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go.

"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.

"We are pulling back after 100% caliphate victory!"

Image: Donald Trump said the IS caliphate is 'ready to fall'

Mr Trump's comments were backed up by Lord Dannatt, the former head of the British Army, who said the UK had a "responsibility" to UK citizens who travelled to Iraq and Syria.

He told Sky News' Sophy Ridge on Sunday show: "If there are - as I think it's correct to say - a large number of foreign fighters in captivity in Syria who originate from countries like the UK, then they are our citizens and we have a responsibility to act responsibly towards them.

"That means they've got to come back to this country, they've got to be held while they're talked to.

"If there is sufficient evidence against any of them, or many of them, they've got to be put through due process and imprisoned if that is the right thing to do.

"It's also important that we treat them fairly and we treat them with justice, but a bit of mercy as well, because I think the way we treat them may well have important significance for the way other people view our society.

"We don't want to see others being radicalised and going off overseas in the future."

The United States is asking Britain, France, Germany and other European allies to take back over 800 ISIS fighters that we captured in Syria and put them on trial. The Caliphate is ready to fall. The alternative is not a good one in that we will be forced to release them........ — Donald J. Trump (@realDonaldTrump) February 17, 2019

....The U.S. does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go. 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. We are pulling back after 100% Caliphate victory! — Donald J. Trump (@realDonaldTrump) February 17, 2019

Culture Secretary Jeremy Wright, who previously served as attorney general, admitted the UK would be "obliged" to take back returning IS fighters.

"It's clear, if you are dealing with a British citizen who wants to return to this country and they're not a dual citizen - so their only citizenship is British citizenship - then we are obliged, at some stage at least, to take them back," he told the BBC's Andrew Marr Show.

"That doesn't mean we can't put in place the necessary security measures to monitor their activities and make sure they are not misbehaving.

"It doesn't mean either that we can't seek to hold them to account for their behaviour thus far.

"But that is a hugely complex problem, it doesn't just involve the UK, it involves many other countries too."

Image: IS bride Shamima Begum wants to return from Syria to the UK

British teenager Shamima Begum is one of those wanting to return to the UK after she fled to Syria to join IS as a schoolgirl in 2015.

She has now given birth to a baby boy in Syria.

Mr Wright added it was a "very different question" as to whether UK officials help facilitate Shamima Begum's return.

Home Secretary Sajid Javid has previously warned he "will not hesitate" to prevent the return of Britons who travelled to join IS.

Gauke: Priority is to protect British public

Mr Trump's comments came after US vice president Mike Pence said the last mile of IS-controlled territory in Syria was being captured by American-backed forces.

The US president's decision to withdraw US troops from Syria has sparked widespread fears that it could lead to a resurgence of IS in the future.

Traumatised IS children being rehabilitated

At the height of the terrorists' power in 2014, the "death cult" controlled nearly a third of Iraq and Syria.

Ciya Furat, a commander with the Kurdish-led Syrian Democratic Forces (SDF), told reporters on Saturday that IS fighters were contained to an area of 700 square metres in the village of Baghuz near the Iraqi border.

See inside an Islamic State house

Thousands of civilians have been streaming out of Baghuz this week to escape the violence.

Activists and residents have warned that IS still has sleeper cells in Syria and Iraq and is laying the groundwork for an insurgency.