Speaking at a high-profile security summit in Singapore, the Pentagon chief also said the US military continues to support diplomats pushing for the "complete, verifiable and irreversible" denuclearisation of the Korean peninsula.

Mattis said Beijing had deployed a range of military hardware -- including anti-ship missiles, surface-to-air missiles and electronic jammers -- across the South China Sea, where it has built islets and other maritime features into hardened military facilities.

Beijing has also landed heavy bombers on Woody Island in the Paracel Islands.

"Despite China's claims to the contrary, the placement of these weapon systems is tied directly to military use for the purposes of intimidation and coercion," Mattis told the Shangri-La Dialogue.