Mr Mattis's comments drew a pointed reaction from a Chinese official at the meeting.
US Defense Secretary James Mattis has accused China of "intimidation and coercion" in the South China Sea, taking a tough stance on Beijing at a security summit in Singapore.
From left, U.S. Defense Secretary Jim Mattis, Japan's Defense Minister Itsunori Onodera and China's People's Liberation Army's Academy of Military Science Deputy President He Lei pose for photos at a ministerial roundtable on the sidelines of the 17th International Institute for Strategic Studies (IISS) Shangri-la Dialogue.
"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", Mattis said.
French Minister of Armed Forces Florence Parly and British Secretary of State for Defence Gavin Williamson issued a joint statement confirming this move at the Shangri-La Dialogue in Singapore.
The comments came on the heels of a string of events that highlight the tension between the world's two biggest economies over the South China Sea's disputed waters.
This month, China's air force landed bombers on disputed islands and reefs in the South China Sea as part of a training exercise.
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The defense secretary said such moves had caused the U.S. to reconsider its "cooperative stance" and disinvite China from the world's largest annual naval exercise later this month.
The Pentagon cited evidence China has deployed anti-ship missiles, surface-to-air missile systems and electronic jammers to contested areas in the Spratly Islands, where China has built military installations on man-made islands.
Beijing's actions were aimed at "national defence", he added in quotes reported by AFP.
He said the United States "will continue to pursue a constructive, results-oriented relationship with China, cooperation when possible, and competing vigorously where we must".
China has always wanted to deal with the South China Sea issue through dialogue and consultation between China and the parties concerned, on the basis of respect for historical facts and in accordance with worldwide law, said the head of the Chinese delegation.
China asserts ownership and control over more than 80 percent of the South China Sea, a US$5 trillion-a-year shipping route where five others including the Philippines and Vietnam have disputed territorial claims. -China relations, as the Trump administration is warily asking for China's help with North Korea ahead of a June 12 summit, while also threatening a trade war.
Mattis also sought to calm nervous allies in the region.
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The Pentagon is developing a more assertive program of so-called "freedom of navigation".
He responded by saying: "The Chinese government and its people strongly oppose arms sale to Taiwan, all kinds of official government contacts to strengthen their ties with Taipei, as well as other acts to harm the "one-China principle".
The Defense Secretary stuck to fairly common talking points from Washington: highlighting the importance of USA alliances and the ultimate goal of complete, verifiable and irreversible denuclearization of the Korean Peninsula.
He said Asia can only prosper under the rule of law, and stressed the importance of a rule-based global order. And he says the US remains committed to ensuring free and open transit in the region.
Using Chinese social media posts, the Washington-based Asia Maritime Transparency initiative identified the location as Woody Island, China's largest base in the Paracel Islands.
"We've been sending a clear message to all that the freedom of navigation is absolutely critical", he said.
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