Donald Trump urges Europe to 'take back' hundreds of captured ISIS fighters

US-backed fighters in Syria poised to capture last ISIS enclave

US President Donald Trump speaks during the Global Coalition to Defeat ISIS meeting at the State Department in Washington. Bloomberg

US President Donald Trump said on Sunday morning the United States was asking European allies to "take back over 800" ISIS fighters captured in Syria and put them on trial.

"The Caliphate is ready to fall," he said in a Tweet. "The alternative is not a good one in that we will be forced to release them..."

US-backed fighters in Syria are poised to capture ISIS's last, tiny enclave on the Euphrates, the battle commander said on Saturday, bringing its self-declared caliphate to the brink of total defeat.

"The US does not want to watch as these ISIS fighters permeate Europe ... 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," he added on Twitter.

The US is assisting SDF fighters in Syria, but plans to withdraw from the country once the last territory held by ISIS is reclaimed.

Mr Trump has frequently criticised European allies for what he considers an inadequate contribution to collective security, most notably over defence spending as a part of Nato obligations.

As well as the approximately 800 ISIS fighters held in Kurdish-run prisons in northern Syria, there are also more than 4,000 family members of captured fighters held in displacement camps.

The issue of what to do with those who left their country to fight for or live in the caliphate was reignited in the UK last week after the news that a girl who left London aged 15 now wants to return home, four years later.

Some of those who have fought for ISIS have been prosecuted and imprisoned, but the process is long and expensive.

Foreign governments fear taking back and imprisoning ISIS fighters because of the security risk they may pose.