United States President Donald Trump on Sunday urged European allies to take back over 800 Islamic State fighters captured in Syria and put them on trial.

Trump’s demand came as US-backed Kurdish forces continued its attack on the last territory held by the Islamic State on the Syrian side of the Iraqi border. The Islamic State fighters are being held by the Kurdish forces.

Trump declared that the “Caliphate was ready to fall”. The US president said if Britain, France, Germany and other European allies did not take the IS fighters back, the alternative would be to release them.

“The US does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go,” he tweeted. “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!”

According to The Guardian, some European countries, especially France, have said they are preparing to take back their former IS fighters. The United Kingdom, however, said fighters held by the Kurdish forces can only return if they seek consular help in Turkey.

US-backed Kurdish forces are clearing Islamic State militants from the last two villages – Shajalah and Baghuz – that the group controls, BBC reported. Both villages are near Syria’s border with Iraq.