US going out of way to cooperate with Pacific nations: Mattis

The US is going out of its way to cooperate with Pacific nations, Secretary of State James Mattis has said, days after America withdrew an invitation to China to participate in the world's largest international maritime warfare exercise.

Asserting that the US is committed to a free and open Indo-Pacific, the Pentagon recently said China's continued militarisation of disputed features in the South China Sea only serve to raise tensions and "destabilise" the region.

Mattis, on his way to Hawaii, the headquarters of the US Pacific Command, told reporters: "We are going out of our way to cooperate with Pacific nations. That's the way we do business in the world".

"But we are also going to confront what we believe is out of step with international law, out of step with international tribunals that have spoken on the issue, and part of this is we maintain a very transparent military activity out in the Pacific," Mattis said in response to a question on the recent Pentagon decision to rescind its invitation to China to participate in the Rim of the Pacific exercise (RIMPAC) military exercise.

"We don't hide it from anyone. We announce it through public affairs statements. The nations, our partner and our allied nations are very open about it. So, when they do things that are opaque to the rest of us, then we cannot cooperate in areas that we would otherwise cooperate in," Mattis said.

Only one country he said is opposing America's freedom of navigation operations.

"You will notice there's only one country that seems to take active steps to rebuff them or state their resentment of them. But it's international waters, and a lot of nations want to see freedom of navigation," Mattis said.

The US will continue with its freedom of navigation operation.

"There had been a promise in 2015 by the -- President Xi (Jinping of China), in the Rose Garden -- the White House meeting where he stated they would not be militarising the Spratly Islands. We have seen, the last month, they have done exactly that, moving weaponry that was never there before," he said.

Withdrawing the invitation to China, the US had cited Beijing's "destabilising behaviour" in the strategically vital South China Sea, signalling fresh strain in bilateral ties.

Held every two years and based in Hawaii, RIMPAC involves more than 20 countries from across the world, including Australia, India, Japan and the United Kingdom.

China claims almost all of the South China Sea but Vietnam, the Philippines, Malaysia, Brunei and Taiwan have counter claims.

The strategic South China Sea is rich in energy reserves, fishery resources and is a busy shipping route.

The Pentagon had called on China to remove the military systems immediately and to reverse course on the militarisation of the disputed South China Sea features.