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 a 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 thatU.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 features the headline "800 jihadis ready to unleash Isil on the West."
Earlier in the week the news that British teen Shamima Begum is 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.
Thousands of people from all over the world were lured to the Middle East by ISIS propaganda to join the militant movement. The jihadist group claimed a caliphate when it emerged in 2014 and took control 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.
"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.