Donald Trump promises announcement on IS caliphate within 24 hours

President Donald Trump on February 15 said the United States would be announcing the end of the Islamic State group's once-sprawling "caliphate" within the next day.

"We have a lot of great announcements having to do with Syria and our success with the eradication of the caliphate and that will be announced over the next 24 hours," Trump told reporters at the White House on February 15.

The US-backed Syrian Democratic Forces have been closing in on holdout jihadists since September last year and a few hundred surviving IS members are now boxed into an area of around one square kilometer (less than half a square mile).

The "caliphate" IS supremo Abu Bakr al-Baghdadi proclaimed in mid-2014 once spanned territory the size of the United Kingdom and administered millions of people.

But successive offensives in Iraq and Syria have shattered the proto-state, which lost its key cities one after the other and has since late 2017 been confined to its traditional power base in the Euphrates valley.

The expected announcement comes after Trump shocked allies and senior figures in his own administration by announcing in December the US was carrying out a full troop withdrawal from Syria.

The plan, which prompted the resignation of then defense secretary Jim Mattis, could be sped up following the declaration of victory.