Delegates hoping for clarity on Trump's intentions for a scheduled June 12 summit with North Korean leader Kim did not get much from Mattis, though he said the issue of the permanent deployment of United States troops in South Korea will not be "on the table".
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.
Critics have said the patrols have little impact on Chinese behavior and mask the lack of a broader strategy to deal with China's growing dominance of the area.
Satellite photographs taken on May 12 showed China appeared to have deployed truck-mounted surface-to-air missiles or anti-ship cruise missiles at Woody, while anti-ship cruise missiles and anti-air missiles were also placed on its largest bases in the Spratlys.
Speaking at the annual Shangri-La Dialogue in Singapore, Mattis said China's placement of weapons systems, including missile batteries and military aircraft, at outposts in the disputed sea raised questions about Beijing's intentions.
"The United States military presence in the South China Sea is greater than that of China and other countries that surround the seas combined", Chinese foreign ministry spokeswoman Hua Chunying said at a regular briefing.
Last month, China's air force landed bombers on disputed islands and reefs in the South China Sea as part of a training exercise, triggering concern from Vietnam and the Philippines.
He warned that America's recent move to disinvite China from a multinational naval exercise this summer was an "initial response" to the militarization of the islands.
But Chinese Lieutenant General He Lei issued a fiery response, telling the summit that "irresponsible comments from other nations can not be accepted".
In response to a question from People's Liberation Army Senior Col. Zhao Xiaozhuo criticizing US freedom of navigation exercises, Mattis said these USA actions were "a reaffirmation of the rules-based order".
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"I understand the disagreement, but it is not one on which we are unstudied", Mattis shot back, noting that the FONOPs take place in global waters.
"Mattis' speech was negative", Zhao said in an interview afterward.
"Nobody is ready to invade those features", Mattis said.
The Chinese officer also slammed Washington's pursuit of closer ties with Taiwan, which Beijing views as the violation of the One-China Policy.
Backing the remarks by his defense secretary, President Donald Trump said on Twitter, "Very surprised that China would be doing this?"
Mr Mattis only made a glancing reference to North Korea, reiterating that it was a diplomatically led effort and the objective remained the "complete verifiable, and irreversible denuclearisation of the Korean Peninsula".
The United States has long demanded that North Korea give up its nuclear weapons.
Vietnamese military officers say they are particularly concerned by the prospect of an ADIZ, saying it could threaten the integrity of Vietnamese airspace.
Jim Mattis said the Pentagon will "compete vigorously" on the issue if needed.
During his trip to Singapore, Mattis has been reluctant to discuss the summit preparations, instead insisting that the Pentagon is focused on backing up US diplomats leading the process.