On a second day in Kuala Lumpur packed with meetings that included multilateral talks with allies Australia, Japan and the Philippines, Hegseth proposed to ASEAN defense ministers the building of shared maritime domain awareness and said China had shown a lack of respect and threatened their territorial sovereignty.

"You live it on the threats we all face from China's aggression and course of actions in the South China Sea and elsewhere," he said.

"We need to develop our joint capabilities to respond, and this includes being able to monitor maritime conduct and develop the tools that allow us to respond quickly [...] ensuring that whoever is on the receiving end of aggression and provocation is then, therefore, by definition, not alone."

"No one can innovate and scale like the United States of America, and we're eager to share those capabilities with allies and partners," Hegseth added.

Hegseth's remarks came a day after the armed forces of Australia, New Zealand, the Philippines and the United States held a drill in the South China Sea, a patrol that a Chinese military spokesperson said "seriously undermined peace and stability".

Beijing claims sovereignty over almost the entire South China Sea via a line on its maps that overlaps with parts of the exclusive economic zones of Brunei, Indonesia, Malaysia, the Philippines and Vietnam.

China has deployed an armada of coast guard vessels hundreds of kilometers off its mainland that has repeatedly clashed with vessels of the Philippines and been accused of disrupting energy activities of Malaysia and Vietnam.

Beijing denies acting aggressively and says its coast guard has operated professionally in defending Chinese territory from incursions.

On Friday, its Defence Minister Dong Jun said it was necessary for China and ASEAN to work together to "pool Eastern strength" and safeguard peace and stability in the South China Sea.

Hegseth's visit to Southeast Asia came soon after US President Donald Trump announced on social media that he had asked the US military to "start testing our Nuclear Weapons", after a halt for 33 years, a move that appeared to be a message to rival nuclear powers China and Russia.

It was not immediately clear whether Trump meant nuclear-explosive testing, which would be carried out by the National Nuclear Security Administration, or flight testing of nuclear-capable missiles.

Asked by reporters what kind of testing Trump was referring to, Hegseth said his department would comment later, adding: "We have very capable nuclear capabilities, and testing them is only prudent."

In addressing the defence ministers' forum, Hegseth touted Washington's peace credentials and said the US was dedicated to building a military "unmatched in global power", while stressing its commitment to allies and partners in the Indo-Pacific.

He said US dialogue with China was important and the opportunity to talk to his Chinese counterpart on Friday was of value, but warned Beijing's actions must be watched closely.

"We seek peace. We do not seek conflict, but we must ensure that China is not seeking to dominate you or anybody else," Hegseth told ASEAN counterparts.