"The global coalition against Daesh has made huge progress, but much remains to be done and we must not lose sight of the threat they pose," Prime Minister Theresa May's spokesman said, using an alternative name for IS.

"Even without territory, Daesh will remain a threat.

"We remain committed to the global coalition and the campaign to deny Daesh territory and ensure its enduring defeat, working alongside our critical regional partners in Syria and beyond."

Repeating a statement issued by the Foreign Office late Wednesday, he said: "This government will continue to do what is necessary to protect the British people and our allies and partners."

Media reports suggested London was not given advance notice of the pull-out.

But the spokesman said: "We have been in discussion with our US partners on this for a number of days."

British Foreign Secretary Jeremy Hunt spoke to US Secretary of State Mike Pompeo on the issue late Wednesday.

Junior defence minister Tobias Ellwood had contradicted Trump on Wednesday, retweeting his message that the jihadists had been defeated in Syria with the words: "I strongly disagree.

"It has morphed into other forms of extremism and the threat is very much alive."

Trump declared on Wednesday that IS had been "beaten" in Syria and announced the pullout of US ground forces from the war-ravaged nation.

Currently, about 2,000 US forces are in Syria, most of them on a train-and-advise mission to support local forces fighting IS.

The Pentagon refused to say what effect the troop withdrawal would have on air operations in Syria that have been ongoing since late 2014.

Britain takes part in the air strikes as part of the US-led anti-IS international coalition.