“Make no mistake: America is in the Indo-Pacific to stay. This is our priority theatre,” he said.

Mattis’ speech came amid increased rivalry between China and the US, as Washington again challenged Beijing’s increasing maritime dominance in the region, mainly over its vast territorial claims and militarisation of islands in strategic waters.

In recent weeks, China has stepped up its campaign with the deployment of advanced weapons and aircraft, including landing its nuclear-capable H-6K strategic bomber on a fortified reef 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 purpose of intimidation and coercion,” Mattis said, adding that Washington would continue to support Association of Southeast Asian Nations members to maintain regional security.

“The more Asean speak with one voice, the more we can maintain the region free from coercion and one that lives by respect for international law.”

He said it was important for the US to uphold alliances with Australia, New Zealand and India and insisted the US was not asking smaller countries to choose between China and the US.

“China should and does have a voice in shaping the international system, and all of China’s neighbours have a voice in shaping China’s role.”

Mattis said also that the US would continue to supply weapons to Taiwan.

“The department of defence remains steadfastly committed to working with Taiwan to provide defence articles and services necessary to maintain sufficient self defence consistent with our obligations set out in our Taiwan Relations Act.”

South Korea and Japan have expressed concern that US President Donald Trump may put America’s security interests before theirs in pursuing a deal with North Korea.

“We are focused on modernising our alliance with both Republic of Korea and Japan, transforming these critical alliances to meet the challenges of the 21st century,” Mattis said.

When asked about the future of US troops in South Korea, he said they were “separate and distinct from negotiations” with North Korea.

Beijing’s drills prompted the US to withdraw China’s invitation to the forthcoming Rim of the Pacific exercises in Hawaii, a move Mattis called a small consequence China faced for “militarising the South China Sea”.

“There will be much larger consequences,” he warned.

He confirmed he would still travel to China to discuss differences between the two nations.

A week earlier, the Indian and Vietnamese navies staged their first joint exercises in the South China Sea, as the two sides seek to enhance cooperation amid China’s growing military presence.

On Wednesday, in what appeared to be a symbolic move signalling India’s growing importance, the US military renamed its Pacific Command the US Indo-Pacific Command.

The command is made up of about 375,000 civilian and military personnel.

While the name change would not involve any fundamental shifts in troops, military missions or other Pentagon activities, Mattis said on Wednesday it was a “recognition of the increasing connectivity between the Indian and Pacific oceans”.

In 2016, the US and India signed an agreement on the use of each other’s land, air and naval bases for repair and resupply, making it easier for them to conduct joint military operations.

The US is also keen to tap into India’s large defence market. It is already New Delhi’s second-largest weapons supplier, closing US$15 billion worth of deals over the past decade.

US congressman Mac Thornberry, who chairs the House Armed Services Committee, said at the Shangri-La Dialogue that the defence bill passed by Washington last week was evidence of America’s concerns in the Asia-Pacific region as it faced an increasingly assertive China.

“That shows not only from the administration, but from Congress, that there is a strong bipartisan commitment to this region,” he said. “There is also a strong bipartisan commitment in Congress to strengthen our military.”

The bill included the suggestion to rename the US Pacific command, as well as recommendations for the US to strengthen partnerships and its own missile defences, and develop the Indo-Pacific stability fund.

A second congressman, Henry Cuellar, said that US policymakers had also been persuading the Philippines to speak up against Beijing’s militarisation in the South China Sea.

“When we were in the Philippines, we told the Filipinos – you can’t just have silence. Because silence means you agree with what China’s [doing],” he said.

“So one of the things we’ve been asking our partners here is you just can’t be quiet. You have to do something, you have to say something.”

The Shangri-La Dialogue, which opened on Friday and closes on Sunday, is an annual security summit for Asia attended by defence ministers and other officials from more than 50 countries, including China, the US, Australia, Japan, India, France, Vietnam and the Philippines.

Additional reporting by Reuters