Beijing's military buildup in the South China Sea calls into question its broader goals in the region, the U.S. secretary of defense told a high-profile worldwide security forum on Saturday.
In a dig at China, which the Pentagon has accused of using "predatory" economics to exploit neighbors, Mattis said the U.S. supports the peaceful resolution of disputes, "free, fair and reciprocal trade and investment" and adherence to worldwide rules and norms.
But "there are much larger consequences in the future when nations lose the rapport of their neighbours", he warned.
On Sunday, the US sailed two Navy ships near islands in the region claimed by China in Washington's latest challenge to Beijing in the sea.
Two US warships sailed close to the Paracel Islands on May 27 in the latest freedom of navigation operation created to challenge Beijing's claims.More news: Blood test offers hope of detecting cancers before symptoms develop
Gen Mattis said Beijing had deployed military hardware, including anti-ship missiles, surface-to-air missiles and electronic jammers to locations across the South China Sea.
In comments at the Shangri-La Dialogue, an global security forum in Singapore over the weekend, Mattis said Beijing's moves were created to intimidate other countries in the region.
Despite his criticism, Gen Mattis added that the U.S. would "continue to pursue a constructive, results-oriented relationship with China" with "co-operation whenever possible". "I believe that what we are going to see is at some point in both Moscow and Beijing they are going to recognize the reality", Mattis said.
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".
He added that the status of U.S. troops in South Korea was not on the table when Mr Trump and Mr Kim meet, but left the door open to the issue being discussed down the road between Seoul and Washington if certain conditions were met. China has not sent high-level officials to the three-day meeting, in an apparent attempt to deflect attention from its campaign to expand its sovereignty across virtually the entire South China Sea.More news: Fallout 76 release date, trailers and news
He also blasted Chinese President Xi Jinping for reneging on a 2015 promise made at the White House that Beijing would not militarise the island features in the South China Sea.
Mr. Mattis said little about North Korea in his speech, which came just hours after Mr. Trump said that the June 12 talks in Singapore with the North's leader, Kim Jong-un, were back on.
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.
Senior Colonel Zhao Xiaozhuo said a U.S. move to send two warships into China's "territorial waters" was a violation of law, and an "obvious provocation to China's national security and territorial integrity". Singapore has warned previously that countries should not be squeezed by competition between the US and a rising China, or forced to take sides.
Mattis has tried to avoid weighing in on the summit, deferring questions to the State Department and Trump's national security team.More news: Trump lawyers' letter to Mueller challenges subpoena
Mattis said the Pentagon will "hold the line" and support the diplomatic effort to secure the "complete, verifiable, and irreversible denuclearization of the Korean Peninsula".