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". Taiwan claims the same area as China.
South Korean Defence Minister Song Young-moo also told the Shangri-La Dialogue security summit that United States forces in South Korea was a "separate issue from North Korea's nuclear issue".
Mr Mattis also touched on Taiwan, a longstanding dispute between the United States and China. He Lei, a deputy president of the Academy of Military Sciences at the People's Liberation Army, headed the Chinese delegation to the Shangri-La Dialogue, and said the disputed islands in the South China Sea are Chinese territory. To reinforce its territorial claims, Beijing has been building artificial islands and deploying military infrastructure on the Paracels, as well as on the Spratly Islands. China claims the self-governing island as its own territory to be brought under its control by force if necessary.
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".
"This is not a revisionist view", Mattis said.
Defense Secretary also emphasized the necessity of expanding cooperation, adding that he would not be wasting time going to Beijing at the end of June if he thought confrontation is the only option regarding relations with China.
Speaking at the annual Shangri-La Dialogue, an Asian security summit in Singapore, Mr Mattis said China's actions were created to intimidate and coerce other nations in the region.
The meetings come following the Trump administration's decision to slap tariffs on allies Canada, Mexico, and the European Union's steel and aluminum shipments to the U.S. Trump has threatened tariffs on up to $150 billion in Chinese products to punish Beijing for pressuring U.S. companies to turn over technology in exchange for access to the Chinese market. "What we see it as, is a reaffirmation of the rules-based order". Mr. Trump announced Friday that the nuclear-weapons summit he had canceled with North Korea's Kim Jong Un is back on.
"Our objective remains the complete, verifiable, and irreversible denuclearization of the Korean Peninsula", Mattis said.
The previous week the US had withdrawn an invitation for Beijing to participate in the exercise known as Rim of the Pacific.
Last month China said it had for the first time landed bombers on Woody Island in the Paracel Islands, prompting USA warnings that it was destabilising the region. China protested the move.
Mr Mattis acknowledged that China's militarisation of islands was a reality but warned of further consequences.
In recent years, the US had sought to stabilize military relations with China, but the militarization of the islands has been a persistent point of conflict.
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 U.S. defense chief accused China of intimidation and coercion in the disputed waters.
China has not sent high-level officials to the Shangri-La conference.