Mattis also highlighted several components of the U.S. Indo-Pacific strategy, pledging to support Asian allies in strengthening the rule of law and protecting maritime borders.
In a speech to the annual IISS Shangri-La Dialogue security forum in Singapore, Ng said the world's two-biggest economies were similarly using security considerations to justify their behavior - the US on trade and China for its military buildup in the disputed South China Sea.
US Defence Secretary James Mattis has raised the prospect of additional American steps against China if its "militarisation" of the South China Sea keeps apace.
He also blasted Chinese President Xi Jinping for reneging on a 2015 promise made at the White House that Beijing would not militarise the island features in the South China Sea.
The Shangri-La Dialogue 2018, or 17th Asia Security Summit, was launched in 2002 by the International Institute for Strategic Studies, a British think tank, and the Singaporean government.
Earlier in the week, Mattis warned the United States will continue "a steady drumbeat" of naval exercises to challenge China's territorial claims in the waterway, and this weekend said, "there are consequences that will continue to come home to roost" if China does not act more transparently and cooperatively in the region.
China recently has deployed anti-ship missiles, surface-to-air missiles, electronic jammers and other equipment on the Spratly Islands, and landed a bomber aircraft at Woody Island.
"The placement of these weapon systems is tied directly to military use for the objective of intimidation and coercion", Mattis said.
Senior Col. Zhao Xiaozhuo said a US move to send two warships into China's "territorial waters" was a violation of law, and an "obvious provocation to China's national security and territorial integrity".
"The U.S. 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. of course we recognise any sustainable Indo-Pacific order has a role for China", he said.
"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.
China asserts ownership and control over more than 80 percent of the South China Sea, a US$5 trillion-a-year shipping route where five others including the Philippines and Vietnam have disputed territorial claims.
He did however, seek to calm nervous allies in the region.
In a dig at China, which the Pentagon has accused of using "predatory" economics to exploit neighbours, Mattis said the United States supports the peaceful resolution of disputes, "free, fair and reciprocal trade and investment" and adherence to global rules and norms. China claims Taiwan as its own and it is one of its most sensitive issues. -North Korea talks will take place.
He also reiterated that the Pentagon was committed to working with Taiwan to provide articles and services needed for its self defence, comments that are likely to anger China.
"We do not do freedom of navigation for America alone, we do freedom of navigation for all nations. we do not see it as a militarization by going through what has traditionally been worldwide water space".
"We have to make it clear that nations need to play by the rules and that there are consequences for not doing so", he added.