However, a Chinese military official dismissed the comments as "irresponsible".
The South China Sea issue has always been a "hot topic" at the annual Shangri-la Dialogue in recent years, so much so that some have described it as a "golden routine subject", said He, who is head of the Chinese delegation to the Dialogue.
He added there would be "consequences" if China does "not find the way to work more collaboratively with all of the nations who have interest".
China has been accused of building artificial military islands in the disputed waterway in an apparent move to intimidate nations with rival claims to territory.
French Minister of Armed Forces Florence Parly and British Secretary of State for Defence Gavin Williamson made the announcement while speaking at the Shangri-La Dialogue in Singapore on Sunday.
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.
On Sunday, two US warships sailed within 12 nautical miles of the disputed Paracel Islands in the South China Sea, drawing a sharp rebuke from China.
Andrei Chang, editor-in-chief of the Canada-based military magazine Kanwa Asian Defence, said Beijing had realised that it was impossible for it to continue to "cover up" the true nature of its plans for the South China Sea reefs.
Mattis said Beijing had deployed a range of military hardware - including anti-ship missiles, surface-to-airmissiles and electronic jammers - across the South China Sea, where it has built islets and other maritime features into hardened military facilities.
"The placement of these weapon systems is tied directly to military use for the objective of intimidation and coercion", Mattis said.
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Last week, two U.S. Navy vessels also sailed near China-controlled islands in what Mattis called "an affirmation of the global rules-based order".
"The US will continue to pursue a constructive, results-oriented relationship with China, cooperating when possible and competing vigorously where we must. of course we recognise any sustainable Indo-Pacific order has a role for China", he said.
Last week, the US withdrew an invitation for Beijing's participation in naval exercises known as the Rim of the Pacific.
Many nations fear that Beijing will use the construction on the islands to extend its military reach and potentially try to restrict navigation in the South China Sea.
He did however, seek to calm nervous allies in the region.
"By exercising our freedom of navigation, we also place ourselves in the position of a persistent objector to the creation of any claim to de facto sovereignty on the islands", she continued.
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.
Senior Colonel Zhao Xiaozhuo, a PLA academic, told Mattis that USA freedom of navigation operations, where U.S. naval vessels sail close to Chinese-claimed features, are an "obvious provocation to China's national security and territorial integrity".
Defense ministers at the Asia Security Summit have welcomed the first meeting of the USA and North Korean leaders scheduled for June 12th.
Mattis said: "We are focussed on modernizing our alliance with both Republic of Korea and Japan, transforming these critical alliances to meet the challenges of the 21st-century".