Mattis specifically called out Beijing's militarization of artificial islands in the South China Sea, home to some of the world's busiest sea lanes.
"Certain countries, under the guise of so-called "freedom of navigation" and 'freedom of aviation, ' have sent military vessels and aircraft to the waters and airspace near China's territory, even sailing within 12 nautical miles of Chinese waters", He said.
He said the USA remained committed to ensuring free and open transit in the region, a hub for shipping and natural resources beset by overlapping claims from China and several of its neighbors including Vietnam and the Philippines.
Washington does not have a claim in the South China Sea but regards it as an worldwide waterway.
In Singapore, Mattis vowed to support USA allies in Asia, "No one nation can and should dominate the Indo-Pacific".
Mattis is again representing the U.S.at this year's forum, which comes amid on-again, off-again prospects for a summit between President Donald Trump and North Korean leader Kim Jong Un this month.
China's activities, Mattis said, are "in stark contrast to the openness our strategy promotes", calling into question "China's broader goals".
Mr Mattis said there was little doubt about Beijing's intentions.
"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 told the Shangri-La Dialogue.
"I think all regional countries with the sharp eyes hope the USA can play a constructive role, and not be a destroyer but a contributor to the regional peace and stability", Hua said.
He and the Philippine defense secretary "emphasized the need" to keep the Indo-Pacific free and open, the website said. "What we see it as, is a reaffirmation of the rules-based order".
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Mattis' comments triggered an equally pointed reaction from a Chinese official at the meeting. Singapore has warned previously that countries should not be squeezed by competition between the US and a rising China, or forced to take sides.
China's hostility towards self-ruled Taiwan has grown since Tsai Ing-wen from the pro-independence Democratic Progressive Party won presidential elections on the island in 2016.
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 global waters.
The US cancelled China's participation in the multinational Rim of the Pacific (RIMPAC) naval drills after China confirmed reports the People's Liberation Army had installed missile systems in the disputed Spratly chain.
The US wanted complete denuclearisation of the Korean peninsula, he added.
Singapore-based security expert Tim Huxley said while increased pressure might slow China's militarization efforts, they would be hard to stop.
Speaking at the same conference, China's Lt Gen He Lei said: "Any irresponsible comments from other countries can not be accepted".
In the latest move, on May 27, U.S. Navy warships sailed near islands occupied by China in the sea's Paracel Islands that sit east of Vietnam.
Mattis made clear that the US does not expect nations to choose between the USA and China, adding that Beijing should have a voice in shaping the region, while allies have a voice in shaping China's role.
The Pentagon chief said Washington will compete vigorously with China's actions if needed.