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A Syria troop pull-out announced by the White House could allow the Islamic State group to rebuild, the Kurdish-dominated force battling the militants on the ground warned on Thursday.

"It will have a negative impact on the counterterrorism campaign," the Syrian Democratic Forces, the US-led coalition's main military partner in the battle against IS, said in a statement.

The SDF, an alliance of Kurdish and Arab fighters which has spearheaded the ground battle against the jihadists, warned a pullout would allow IS to regroup and launch a counterattack.

"It will give terrorism... the momentum to recover and conduct a terrorist campaign in the region," the statement said, creating a “political and military vacuum in the area, leaving its people between the claws of hostile parties."

In an announcement that appeared to take Syria's Kurds by surprise, US President Donald Trump on Wednesday declared victory over IS in Syria and announced a full troop pull-out.

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

"We've won against ISIS," Trump, said using another acronym for the jihadist group, in a short video posted on social media.

His claim contradicted even members of his own camp and coincided with warnings from officials and analysts that IS had recently managed to resurface in various parts of Syria and Iraq.

The decision runs counter to long-established US policy for Syria and the region. It blindsided lawmakers, the Pentagon and international allies alike.

Britain and France warned on Thursday that the fight against jihadists in Syria was not finished.

A statement issued by the British government, which has long supported the anti-IS campaign in Syria, said "much remains to be done" against the jihadists.

"We must not lose sight of the threat they pose. Even without territory, (IS) will remain a threat," the statement read.

Junior defence minister Tobias Ellwood was more blunt, retweeting a message from Trump 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."

France said Thursday it will maintain its participation in the coalition fighting IS forces in Syria. European Affairs Minister Nathalie Loiseau said "the fight against terrorism is not over."

Agencies contributed to this report.