Late on Thursday, Chinese state media reported that the country's defence minister had urged the USA to "walk with China" to maintain stability and peace in the disputed waters at a meeting with his American counterpart at the summit.
The much-anticipated meeting between the Pentagon chief and General Wei Fenghe on the sidelines of a regional security summit in Singapore is expected to see the men pursue more durable military relations that can better withstand political and economic crises.
The Chinese government requested the meeting, according to senior Department of Defense officials traveling in Asia with Mattis. "It is an important way for (Chinese President) Xi Jinping to signal to his domestic population that he has a handle of the US-China relationship, that it's not spiraling out of control".
By working with ASEAN and other partners, the US affirms that "no single nation can rewrite the worldwide rules of the road", he added.
Mattis said he remains keen for a "constructive relationship" with China but expressed concern about its military activities in the disputed South China Sea.
China has been infuriated by the United States putting sanctions on China's military for buying weapons from Russian Federation, and by what Beijing sees as stepped-up USA support for self-ruled Taiwan, claimed by China as its sacred territory.
Mattis, speaking to reporters as he flew to Asia this week, rejected Chinese claims that the United States was acting aggressively and pointed the finger at Beijing.
U.S. officials have long expressed dismay at China's construction of industrial outposts and military facilities on artificial islands in the South China Sea but have mostly reserved their reactions to verbal outbursts.
"We need to make sure that when we step on one another's toes it doesn't escalate into something that would be catastrophic", he said.
As part of this, the navies of China and the 10-member Association of Southeast Asian Nations, or ASEAN, are set to hold their first joint drills, which will take place in the South China Sea.
Mattis has made eight trips to what he calls the Indo-Pacific region as Pentagon chief, and a primary mission has been to encourage countries to stand up to China's vast claims of sovereignty in the strategically vital South China Sea.
"The US is keen to develop military-to-military relations, and [we] think increasing cooperation is the only reasonable way to develop the relations".
In an apparent effort to lessen any such fears, Ng also said Asean was planning to hold maritime exercises with the USA for the first time next year.
Since then, however, a series of irritants have shaken military-to-military ties.
Beijing appears ready to normalise its interactions with the American military, a United States defence official said Wednesday, after relations soured during a sanctions spat and trade war.