The much-anticipated meeting between the Pentagon chief and General Wei Fenghe saw the men address long-standing frictions -- including Beijing's actions in the South China Sea -- but also centred on strengthening ties to withstand political crises like the one currently engulfing the two nuclear powers.

Mattis "repeated our desire for a durable relationship that is a stabilising force in the overall relationship," Randall Schriver, the Pentagon's assistant secretary of defence for Asian and Pacific Security Affairs, told reporters after the nearly 90-minute meeting on the sidelines of a security summit in Singapore.

"When we have times of differences and irritants, we should seek to deepen our contact, particularly at the high level, strategic level so that we can talk through differences."

A meeting between Mattis and Wei was supposed to take place in Beijing last weekend, but it fell through after China declined to make Wei available.

The snub came as China reacted angrily after Washington imposed sanctions following Beijing's purchase of Russian fighter jets and missiles.

Beijing's other actions included scrapping a planned port visit of a US warship to Hong Kong and cancelling a meeting between the head of the Chinese navy and his American counterpart.

And tensions reached a dangerous level when a Chinese warship sailed extremely close to a US destroyer as it conducted a "freedom of navigation" operation challenging China's extensive sovereignty claims in the South China Sea.

The contested waterway, where China has built a series of military facilities on islets and reefs, was a subject of "significant" discussion, Schriver said, but no progress was made in resolving the issue or the other perennial point of friction -- Taiwan.

"We're going to continue to have differences," Schriver said.

"They weren't resolved at this meeting and during this discussion and they probably won't be in a next discussion. There will be issues that are long-term challenges to manage."

- Tough rhetoric -

Mattis's attempts to carve a better relationship with the Chinese military stands in contrast to the anti-Chinese rhetoric from the White House.

President Donald Trump has frequently assailed China over its economic policies and earlier this month, US Vice President Mike Pence issued a litany of complaints, accusing Beijing of "predatory" trade practices and military "aggression", among other charges.

Mattis has made eight trips to the Asia region as Pentagon chief, and a primary mission has been to encourage countries to stand up to China in the South China Sea.

But Schriver indicated that countries remain too intimidated to do much on this front.

"In some instances, other countries may not have the confidence given China's strength to always speak up, but (Mattis) wanted to let Minister Wei know that he hears about it a great deal from other countries," Schriver said.

Mattis's visit to Singapore comes as questions swirl in Washington about his future.

In an interview broadcast Sunday, Trump suggested Mattis may be headed out the Pentagon door.

"It could be that he is. I think he's sort of a Democrat, if you want to know the truth," Trump said.

Mattis later told reporters that Trump had assured him of his "100 percent" support.

Gregory Poling, an Asia expert and fellow at the Center for Strategic and International Studies think-tank, said most countries in the region see Mattis as "a steady hand on the tiller".

But they are wary of the uncertainty in Washington and Trump's mercurial approach to America maintaining a leadership role in the region.

Once Mattis leaves after a trip, "officials start wondering... is the full force of American power really in Asia to stay?" Poling told AFP.

"I don't think Asian leaders see lasting commitment from the US on things other than North Korea and trade deficits."

Schriver said the issue of North Korea came up with Wei "in very general terms" as a possible area to deepen cooperation.

"We do have a shared view that we would like to see denuclearisation occur," he said.