President Donald Trump has demanded that the European nations take back more than 800 ISIS fighters captured in Syria and put them on trial, warning that the US otherwise will be forced to free the jihadists after it pulls out from the war-torn country.

He also warned the Islamic State (ISIS) that America would hit them hard from nearby military bases if the terror outfit regains momentum.

US-backed fighters are set to capture ISIS's last tiny stronghold in Syria, bringing an end to its self-declared caliphate.

"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," Trump said on Saturday.

President Trump warned that ISIS fighters could "permeate Europe" and called on European nations to "step up" efforts to put the prisoners on trial in their countries.

"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!" he said in a tweet.

ISIS, which once controlled huge swatches of land in Iraq and Syria, suffered crippling defeats in 2017 when Iraq recaptured Mosul and the Syrian Democratic Forces seized its Syrian capital of Raqqa, and the Syrian government pushed it east to the Euphrates.

Trump has said he will pull US forces from Syria after ISIS' territorial defeat, raising questions over the fate of the US' Kurdish allies vulnerable to an attack from Turkey.

Ankara views the Kurdish forces as terrorists aligned with insurgents inside Turkey.

Last month, Trump threatened to "devastate" Turkey economically if the NATO-allied nation attacks US-backed Kurdish forces in Syria following a pullout of American troops from the war-torn country. He also urged the Kurds not to "provoke" Ankara.

Ground troops first arrived in Syria in autumn 2015 when then US President Barack Obama sent in a small number of special forces to train and advise YPG fighters.

A peaceful uprising against the president of Syrian President Bashar al-Assad seven years ago turned into a full-scale civil war. The conflict has left more than 350,000 people dead, devastated cities and drawn in other countries.

(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)