"Certainly in conversations this past week with my British and French counterparts, I'm very optimistic that they're going to participate," John Bolton said, in an interview with ABC's "This Week."

"It hasn't happened formally yet, but they're looking at it," he said, adding that the chairman of the Joint Chiefs of Staff, Joe Dunford, is working to set up the force.

Bolton insisted there was no contradiction between Trump's assertion that the caliphate declared by the Islamic State group has been eliminated 100 percent, and the assessment of the top US commander in the Middle East, who told Congress last week the fight is "far from over."

"The president has been, I think, as clear as clear can be when he talks about the defeat of the ISIS territorial caliphate," Bolton said.

"He has never said that the elimination of the territorial caliphate means the end of ISIS in total. We know that's not the case."

"But one reason that the president has committed to keeping an American presence in Iraq and a small part of an observer force in Syria is against the possibility that there would be a real resurgence of ISIS, and we would then have the ability to deal with that if that arose."

Trump abruptly announced in December the immediate and complete withdrawal of the 2,000 US troops deployed in northeastern Syria, declaring victory against IS, or ISIS as it is also known.

Then, under pressure from Congress and the Pentagon, he agreed to leave a residual force of some 200 US troops, which he wants to be reinforced by allies in the anti-IS coalition.

An objective of the international force is to guarantee the security of its Syrian Kurd allies. Turkey, a NATO member, views the Kurdish combatants as terrorists, and the Europeans fear they would be vulnerable if Ankara launched an offensive.