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 fact that Mattis "is meeting with Minister Wei is some evidence that the Chinese are interested in keeping things normal and stable -- as are we," Randall Schriver, the Pentagon's assistant secretary of defence for Asian and Pacific Security Affairs, told reporters.

"We need to make sure that when we step on one another's toes it doesn't escalate into something that would be catastrophic."

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.

China had reacted angrily after Washington imposed sanctions following Beijing's purchase of Russian fighter jets and missiles, and a close encounter between warships in the South China Sea raised the spectre of a mishap.

Beijing's other actions have 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.

If Mattis does strike a conciliatory tone, it will stand 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.

- 'Sort of a Democrat' -

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.

Schriver indicated that countries aren't doing enough on this front, and said Mattis would be encouraging them to assert a presence in the areas which Beijing claims.

"We will fly sail and operate where international law allows but we are also looking for partners to give voice to keeping... international law... upheld," he said.

Mattis's visit 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, 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."