WASHINGTON (REUTERS) - United States President Donald Trump has demanded that European allies take back more than 800 ISIS fighters captured in Syria and put them on trial.

"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," he said in a tweet.

"The Caliphate is ready to fall. The alternative is not a good one in that we will be forced to release them... The US does not want to watch as these ISIS fighters permeate Europe, which is where they are expected to go," Mr Trump said.

"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!"

Mr Trump has sworn to pull US forces from Syria after the Islamic State in Iraq and Syria's territorial defeat, raising questions over the fate of Washington's Kurdish allies and Turkish involvement in north-east Syria.

US-backed fighters in Syria are poised to capture ISIS' last, tiny enclave on the Euphrates, the battle commander said on Saturday (Feb 16), bringing its self-declared caliphate to the brink of total defeat.

Commander Jiya Furat said the Syrian Democratic Forces (SDF) had cornered the remaining militants in a neighbourhood of Baghouz village near the Iraqi border, under fire from all sides.

"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, using the Arabic acronym for ISIS.

He was speaking after Mr Trump said last Friday that there would be "great announcements" about Syria over the next 24 hours.

As the SDF advanced under heavy US airstrikes in recent days, a stream of civilians fled the few square miles of hamlets and farmland that remain within ISIS' "caliphate", along with defeated militants trying to escape unnoticed.

Though ISIS fighters still hold out in a pocket of central Syria's remote desert, and have gone underground as sleeper cells in Iraqi cities, their territorial rule is, for now, almost over.

It ends a project launched from the great mediaeval mosque of Mosul in northern Iraq in 2014, when Abu Bakr al-Baghdadi seized advantage of regional chaos to proclaim himself caliph, suzerain over all Muslim people and land.

He set up a governing system with courts, a currency and flag that at its height stretched from north-west Syria almost to Baghdad, encompassing some two million inhabitants.

But its reign of terror over minorities and other perceived enemies, marked by massacres, sexual slavery and the beheading of hostages, drew a forceful international military response from 2015 that pushed it steadily back.

Most of the fighters left in Baghouz are foreigners, the SDF has said, among the thousands drawn by Baghdadi's promise of a new extremist utopia straddling the Iraqi-Syrian border and expunging national borders.

All that remains, said Commander Furat, is an encircled pocket of some 700 sq m. "Thousands of civilians are still trapped there as human shields," he said.

The Britain-based Syrian Observatory for Human Rights said the SDF had taken control of all of Baghouz after militants there surrendered. SDF officials denied this.

Spokesman Mustafa Bali said the SDF had caught several militants trying to flee among the civilians. Others had handed themselves over.

Their fate, and that of their families, has befuddled foreign governments, with few ready to repatriate citizens who pledged allegiance to a group sworn to their destruction, but who might be hard to legally prosecute. The SDF does not want to hold them indefinitely.

The fate of Baghdadi is also a mystery. He has led the group since 2010, when it was still an underground Al-Qaeda offshoot in Iraq.

Its capacity for strategic retreats in hard times, followed by rebounds when circumstances changed, has prompted numerous warnings that ISIS' defeat has not ended the threat it poses to the region.

ISIS suffered crippling defeats in 2017, when Iraq recaptured Mosul, the SDF seized its Syrian capital of Raqqa, and the Damascus government pushed it east to the Euphrates.

But in Iraq, it has switched to guerrilla hit-and-run tactics, aimed at undermining the Baghdad government. It has also claimed responsibility for a series of bombings in swathes of north-east Syria held by the SDF, including one last month that killed four Americans.

That attack came soon after Mr Trump, rattling allies and prompting Defence Secretary Jim Mattis to resign, pledged to pull out, saying that ISIS was already defeated.

Turkey, which regards the SDF's strongest component, the Kurdish YPG, as terrorists, has threatened to march deeper into northern Syria to drive it back.

Last Friday, US Army General Joseph Votel, who oversees US forces in the Middle East as head of Central Command, said the end of the territorial caliphate would lead to a more dispersed, harder-to-detect network of fighters waging guerrilla warfare.

That should require continued help from Washington, he said.