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Trump calls on U.S. allies in Europe to take back 800 captured ISIS fighters

"The U.S. does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go," Trump warned.

LONDON — President Donald Trump called on European countries to take back captured Islamic State fighters late Saturday as U.S.-backed forces in Syria closed in on the extremist group's final sliver of territory.

"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," Trump said on Twitter.

"The caliphate is ready to fall," he added.

After years of global effort to combat the group, the U.S.-backed Syrian Democratic forces have cornered remaining militants in a village near the Iraqi border, under fire from all sides.

The final assault has been prolonged because ISIS fighters have been using civilians as human shields, SDF spokesman Mustafa Bali told NBC News.

"In the coming few days, in a very short time, we will spread the good tidings to the world of the military end of Daesh," he said Saturday, using the Arabic acronym for Islamic State.

Trump announced in December that U.S. troops would be leaving Syria, a sudden policy shift that blindsided U.S. allies as well as many in Washington.

He reiterated that stance Saturday, urging European countries to do more as the U.S. withdraws and suggesting the captured fighters would otherwise be released.

"The U.S. does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go," Trump warned.

Even before Trump's tweets, the potential threat posed by those left behind once ISIS loses its final stranglehold on territory had struck a nerve in Europe.

Sunday's front page of the U.K.'s Telegraph newspaper featured the headline "800 jihadis ready to unleash Isil on the West."

NBC News reported recently that France is accelerating plans to end its military commitment in Syria and is considering airlifting captured foreign ISIS fighters out of the country, fearing a precipitous American withdrawal from the battlefield will leave liberated areas unstable and make it impossible to contain the prisoners.

“Syrian Democratic Forces are currently holding foreign terrorist fighters, including French nationals, in northeastern Syria,” said Agnès Von Der Mühll, a foreign ministry spokesperson.

She would not confirm the number of detainees, however, adding that in light of “American decisions,” the French government was “exploring all options in order to prevent these potentially dangerous individuals from escaping or dispersing.”

The U.S. special envoy on Syria sought to reassure allies at the Munich Security Conference on Sunday.

"We've been telling them (allies) continuously this is not going to be an abrupt, rapid withdrawal but a step-by-step withdrawal," said James Franklin Jeffrey.

Law enforcement agencies from around the world have been working to share information about captured fighters and terror suspects, often through Interpol, the global law enforcement organization that is headquartered in France.

Thousands of foreign nationals were lured to the Middle East to join the militant movement when it emerged in 2014 and took control of vast swathes of territory stretching across both Syria and Iraq.

But accepting and prosecuting returnees who committed crimes isn't a simple task. Not all Westerners who joined the group were fighters and the depth of their involvement isn't always clear.

Earlier in the week the news that British teen Shamima Begum was now pregnant in a refugee camp and asking to come home after running away to Syria in 2015 to join ISIS set off a fierce public debate over whether she and others like her can be rehabilitated.

The news Sunday that Begum had now reportedly given birth to the baby, from whom she had pleaded not to be separated if she does return to Britain, could further complicate matters.

"Somebody who has spent a lot of time in the caliphate is likely to be radicalized, and women are as capable of committing terrorist acts as men," said NBC News security analyst Duncan Gardham of Begum. "It may be a difficult task to make sure she is not radicalized and ensure that she's not a threat."

For those who did commit crimes, there are considerations to be made of where to incarcerate them — without risking radicalizing other inmates — and what to do with them upon their release, Shiraz Maher, director of the International Centre for the Study of Radicalisation and Political Violence, at Kings College London, said in a tweet thread on Thursday.