US Defense Secretary warns of 'tough fight' to oust IS

The US-led military alliance battling the Islamic State group faces "a tough fight" to oust the jihadists from their last holdouts in Syria, US Defense Secretary Jim Mattis said Tuesday.

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."