ABOARD A US MILITARY AIRCRAFT • US Defence Secretary James Mattis said that the United States would continue to confront what Washington sees as China's militarisation of islands in the South China Sea, despite drawing condemnation from Beijing for an operation in the region over the weekend.

Reuters first reported that two US Navy warships sailed near South China Sea islands claimed by China on Sunday, even as President Donald Trump seeks Chinese cooperation on North Korea.

The "freedom of navigation" operation was the latest attempt to counter what Washington sees as Beijing's efforts to limit freedom of navigation in the strategic waters, where Chinese, Japanese and some South-east Asian navies operate.

China expressed its anger, saying it had sent ships and aircraft to warn the US warships to leave.

"You'll notice there is only one country that seems to take active steps to rebuff them or state their resentment (to) them, but it's international waters and a lot of nations want to see freedom of navigation," Mr Mattis told reporters on Tuesday while en route to Hawaii, where he will oversee a change of command for US Pacific Command.

Satellite pictures showed China seemed to have deployed truck-mounted surface-to-air missiles or anti-ship cruise missiles at Woody Island. Mr Mattis, who will be in Singapore for the annual IISS Shangri-La Dialogue, said he was planning to raise US concerns about China's recent moves to "militarise" the South China Sea.

Earlier this month, China's air force landed bombers on disputed islands and reefs in the South China Sea as part of a training exercise in the region.

"When they (Chinese) do things that are opaque to the rest of us, then we cannot cooperate in areas that we would otherwise cooperate in," Mr Mattis said. US diplomats were engaged on the issue and he had heard concerns about Chinese actions not just from within the US government, but also from regional allies, he added.

REUTERS, BLOOMBERG