US Defence Secretary James Mattis has raised the prospect of additional American steps against China if its "militarisation" of the South China Sea keeps apace.
But "there are much larger consequences in the future when nations lose the rapport of their neighbors", he warned.
Mattis told the IISS Shangri-La Dialogue security conference in Singapore that China's had recently sent missile batteries, military aircraft and communications-jamming equipment to its outposts in disputed areas of the key shipping route.
Last 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, triggering concern from Vietnam and the Philippines.
"Beijing has deployed a range of military hardware including anti-ship missiles, surface-to-air missiles and electronic jammers across the South China Sea, where it has built islets and other maritime features into hardened military facilities", Mattis said.
A Chinese general on Saturday, June 2, lashed out at "irresponsible comments" on Beijing's military build-up in the South China Sea after the United States defense chief accused China of intimidation and coercion in the disputed waters. It says military activities on the islands have been for self-defense purposes.
The president's tweet comes after the White House announced the official US delegation to China for a series of upcoming meetings slated to be held next week, following talks held in Beijing and Washington last month.
The Pentagon does not comment on future operations but a spokesman, Lieutenant Colonel Christopher Logan, said "we will continue to work with our friends, partners, and allies to ensure a free and open Indo-Pacific".
Mattis said any discussion about the future of 28,000 USA troops now stationed in South Korea "will be separate and distinct from the negotiations" with North Korea.
The previous week the USA had withdrawn an invitation for Beijing to participate in the exercise known as Rim of the Pacific.
Critics have said the patrols have little impact on Chinese behavior and mask the lack of a broader strategy to deal with China's growing dominance of the area. Mattis characterized this action as an "initial response".
Singapore-based security expert Tim Huxley said while increased pressure might slow China's militarization efforts, they would be hard to stop.
"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".
On Sunday, two U.S. warships sailed within 12 miles of islands in the South China Sea in an attempt to assert the USA stance on freedom of navigation in worldwide waters.
Mattis's comments appeared to be an attempt to reaffirm the USA commitment to Asia's security, even as Trump's decisions to levy tariffs on allies and withdraw from global agreements raise questions about America's postwar alliances. Mr. Trump announced Friday that the nuclear-weapons summit he had canceled with North Korea's Kim Jong Un is back on.
"By exercising our freedom of navigation, we also place ourselves in the position of a persistent objector to the creation of any claim to de facto sovereignty on the islands", she said.
Mattis only made a glancing reference to North Korea, reiterating that it was a diplomatically led effort and the objective remained the "complete verifiable, and irreversible denuclearisation of the Korean Peninsula".