US to confront China over islands: Mattis

US Defence Secretary Jim Mattis says the US will continue to confront China's militarisation of man-made islands in the South China Sea.

Mattis says Beijing hasn't abided by its promise not to put weapons on the Spratly Islands.

He said on Tuesday that American ships are maintaining a "steady drumbeat" of naval operations around disputed islands, and "only one country" seems to be bothered by the vessels' activities.

Mattis said the US would confront "what we believe is out of step with international law."

During the weekend, two US warships sailed close to the Paracel Islands, north of the Spratlys, drawing protests from China.