China is guilty of "intimidation and coercion" over its military build-up on disputed islands in the South China Sea, the US defence secretary has said.

James Mattis said China's actions in the region stood in "stark contrast" to the US policy of openness and called into question China's goals.

The area has become the focus of increased tensions after China appeared to install anti-aircraft and anti-missile systems on seven artificial platforms it has constructed on atolls in the Spratly Islands.

It comes days after Philippine President Rodrigo Duterte was said to have threatened war if his troops were harmed in the area, which his country also lays claim to.

Image: US defence secretary Jim Mattis addresses the Shangri-La Dialogue in Singapore

Mr Mattis, speaking at the annual Shangri-La Dialogue summit in Singapore, said: "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.


"China's policy in the South China Sea stands in stark contrast to the openness that our strategy promotes, it calls into question China's broader goals.

"The US will continue to pursue a constructive, results-oriented relationship with China. Co-operation whenever possible will be the name of the game and competing vigorously where we must... of course we recognise any sustainable Indo-Pacific order has a role for China."

Image: An AMTI image apparently showing anti-aircraft guns on Hughes Reef in the South China Sea

China accused the US of trespassing after two Navy warships sailed near a reef claimed by Beijing on Sunday.

The incident was the first time Donald Trump's administration challenged Chinese claims to the disputed waters but Mr Mattis' speech represents a step up in the war of words.

A study by Asia Maritime Transparency Initiative (AMTI) found last year that China had built military length airstrips on the previously uninhabited Fiery Cross, Mischief and Subi reefs in the Spratlys.

Satellite images of the Hughes and Gaven reefs also showed what appeared to be anti-aircraft guns and what were likely to be close-in weapons systems (CIWS) to protect against cruise missile strikes.

Image: An AMTI image apparently showing anti-aircraft guns on Subi Reef

The archipelago, named after British whaling captain Richard Spratly, lies between Philippines, Malaysia, Vietnam and Brunei and parts of it are claimed by all.

Taiwan and China, which are hundreds of miles to the north, also claim some of the islands.

China's state-run Global Times newspaper said in an editorial that tension in the South China Sea was "due to the US continuing to increase its military presence in the region, forcing China to naturally upgrade its defensive weapons on the islands."

It said: "This in turn gives the US more excuses to exert military pressure, causing regional tensions to spiral."

Image: An AMTI image apparently showing anti-aircraft guns on Fiery Cross Reef in the South China Sea

Mr Mattis acknowledged that militarisation of islands was taking place but warned of further consequences.

"I believe there are much larger consequences in the future when nations lose the rapport of their neighbours... eventually these (actions) do not pay off," he said.

The defence secretary also confirmed the Pentagon was keen to help Taiwan defend itself, in comments that were likely to further anger China, which also claims the island.