While the extremists have lost almost all of the self-declared "caliphate" they held across Iraq and Syria four years ago, Mattis warned that destroying the group completely was "still going to take some time".

"Make no mistake about it, as ISIS has collapsed inward, in their own way they have reinforced the centre as they've been forced into what is now less than two percent of their original territory," Mattis told reporters in Paris.

"So it's going to still be a tough fight, I don't want anyone to be under any illusions," he added.

"We will be successful but it's still going to take some time."

Last month the military chief of France, which has troops in the US alliance fighting IS, estimated the jihadists would lose their last Syrian territory by January.

Francois Lecointre predicted "the end of the physical caliphate of Daesh before the end of the year, probably late autumn", using another name for IS.

IS has lost all of its urban centres in Iraq and late last month US-backed Kurdish and Arab fighters launched a fresh operation to oust the group from its last holdouts in southeast Syria.