The defense secretary said such moves had caused the United States to reconsider its "cooperative stance" and disinvite China from the world's largest annual naval exercise later this month.
"It would be a lose-lose scenario for the world if the USA and China are unwilling to work together for an inclusive system that both large and small states can benefit from, and where rules apply to all", Ng said.
"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", Gen Mattis said. And he says the USA remains committed to ensuring free and open transit in the region.
USA assertions that China is militarising the are "ridiculous", China said on Thursday, after Secretary said Washington would confront China's actions in the disputed waterways.
Beijing's series of recent provocations in the disputed waterway, which include the installation of anti-ship cruise missiles and radar jamming equipment on several of its outposts - in addition to the landing of bomber aircraft on Woody Island - has triggered concern among neighboring countries who also lay claim to the area.
The area has become the focus of increased tensions after China appeared to install anti-aircraft and anti-missile systems on seven artificial platforms it has constructed on atolls in the Spratly Islands.
"I believe there are much larger consequences in the future, when nations lose the rapport of their neighbours", Mattis said after his speech in response to a question from the media.
Responding to Mattis' hardline talks, the Chinese delegates lashed out at the "irresponsible comments".
The bluster comes at a delicate time in US-China relations, as the Trump administration is warily asking for China's help with North Korea ahead of a June 12 summit, while also threatening a trade war.
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Mattis responded that the question reflected a fundamental disconnect with the way global tribunals have spoken on the matter.
Hours after Mattis spoke, Chinese Lieutenant General He Lei lashed out at "irresponsible comments" on China's build-up in the South China Sea. "We see it as affirmation of the rules-based worldwide order". Trump announced Friday that the meeting was back on. The summit is expected to be here in Singapore. He said he would visit Beijing soon at China's invitation.
Despite strong mutual suspicions, the United States had included China in the past two versions of the naval exercise known as Rim of the Pacific, or RimPac, in 2014 and 2016.
"China should and does have a voice in shaping the worldwide system, and all of China's neighbors have a voice in shaping China's role", said Mattis, adding that he would travel to Beijing soon "at China's invitation".
Speaking at the same conference, China's Lt Gen He Lei said: "Any irresponsible comments from other countries can not be accepted".
China has repeatedly warned the USA against sending warships to patrol the South China Sea. It was the latest freedom of navigation operation created to challenge Beijing's claims. China protested the move. "Our Indo-Pacific strategy informs our relationship with China".
"The Chinese People's Liberation Army has the determination, confidence and ability to safeguard China's sovereignty, security, unity and development interest".
"We have seen those who wanted to dominate the region come and go, and we have been with you", Mattis said.
"If the USA will continue to pursue a constructive results-oriented relationship with China, cooperation, whenever possible, will be the name of the game, and competing vigorously where we must".