China has indisputable sovereignty over the islands in the South China Sea and the adjacent waters, a fact verified by historical evidence and supported by worldwide law, he said.
Speaking at the same conference in Singapore, Lt Gen He Lei said, "Any irresponsible comments from other countries can not be accepted".
The US is considering intensified naval patrols in the South China Sea in a bid to challenge China's growing militarisation.
He said on Saturday that "to muscle their way in is not the way to make long-term collaboration in a region that is important to China", referring to China's "militarisation of artificial islands in the South China" and its competing claims with Vietnam, the Philippines, Taiwan (region), Malaysia and Brunei over islands there.
Pentagon officials have long complained that China has not been candid enough about its rapid military build-up and using South China Sea islands to gather intelligence.
The Pentagon cited strong evidence that China has deployed anti-ship missiles, surface-to-air missile systems and electronic jammers to contested areas in the Spratly Islands.
"Despite China's claims to the contrary, the placement of these weapon systems is tied directly to military use for the purposes of intimidation and coercion", Mattis said.
This month, China's air force landed bombers on disputed islands and reefs in the South China Sea as part of a training exercise.
The Pentagon is weighing a more assertive programme of the so-called freedom-of-navigation operations close to Chinese installations on disputed reefs, two U.S. officials and Western and Asian diplomats close to the discussions said.
South Korea and Japan have expressed concern that Trump may put USA security interests ahead of theirs in pursuing a deal with North Korea.
While the operation had been planned months in advance, and similar operations have become routine, it is believed to be the first time where two United States warships have been used for a freedom of navigation operation in the South China Sea.
He called the U.S. action a "relatively small consequence".
China claims its actions in the South China Sea are entirely peaceful and meant to protect its citizens and trading interests. Trump's recent tariffs on steel and aluminum imports have targeted allies in Europe, plus Japan, Canada and Mexico, while the U.S.is engaged in an escalating war of words with China, risking a full-blown trade war.
Mattis emphasized that the US military wants more transparency from China.
Under President Donald Trump's administration, Washington has made "a free and open Indo-Pacific" - a term replacing the more widely used "Asia Pacific" label to acknowledge a role played by India - the cornerstone of its foreign policy in the continent.
France and the UK's commitment is in line with the USA's plan to ramp up freedom of navigation operations in the region.
Mattis also touched upon the status of Taiwan, an issue bound to ruffle feathers in Beijing. "If China's islands and reefs are continuously threatened by activities under the name of so-called freedom of navigation, China will eventually station troops on these reefs".
Beijing has been accused of ramping up the pressure on Taipei in recent weeks, with Taiwan accusing using its diplomatic and economic weight to isolate the island from the global community.
The China-proposed Belt and Road Initiative is not only a path of development but also a path of peace, as it will not only bring opportunities to the economic development of regional countries, but also provide ideas and solutions for them to solve security problems, he noted.
The Pentagon chief said the United States was "steadfastly committed to working with Taiwan to provide defence articles" consistent with the "obligations set out in our Taiwan Relations Act".