US Defence Secretary James Mattis will arrive in Ho Chi Minh City on his second trip to Vietnam this year amid rising tensions with China over trade.

Mattis, a retired Marine general and former commander of US troops in Iraq and Afghanistan, will land in Ho Chi Minh City on Tuesday afternoon and Ho Chi Minh City Communist Party boss and de-facto mayor Nguyen Thien Nhan will greet him upon landing.

Mattis is also scheduled to visit Bien Hoa airbase on Wednesday during his two-day trip and meet Defence Minister Ngo Xuan Lich.

The base, located north of Ho Chi Minh City, is the site of a US-sponsored clean-up of Agent Orange, an herbicide used by the US during the war that has since been blamed for hundreds of thousands of birth defects.

Mattis last visited Vietnam in January, when he held talks with Lich in Hanoi. The trip was followed by the visit of the USS Carl Vinson aircraft carrier in March to Da Nang.

Carl Thayer, an emeritus professor at the University of New South Wales and an expert on south-east Asia, said Mattis was likely to discuss Agent Orange clean-up, support for Vietnamese peacekeepers in South Sudan and Vietnam's purchase of Russian weapons, which may run afoul of US sanctions against Moscow.

"Vietnam will come under increased pressure to step up defence cooperation with the United States including more frequent naval port visits," Thayer added.

The US and Vietnam have enjoyed increasingly close relations in recent years as both countries share concerns about Chinese maritime claims in the South China Sea.

China claims almost the entire South China Sea, including waters internationally recognized as Vietnam's exclusive economic zone.

Mattis was to also visit Beijing during his current Asia trip, but the trip was cancelled as the two countries' trade dispute brews.

US President Donald Trump last month slapped additional tariffs on Chinese goods, after imposing duties on $US50 billion worth of imports earlier this year. China has retaliated with its own duties on US imports including soybeans, cars and aircraft.

The trip comes amid rumors from the White House that Mattis may resign.

Trump said Mattis was "a good guy" but "sort of a Democrat, if you want to know the truth," in excerpts released ahead of an interview released Sunday on the CBS programme 60 Minutes.

While travelling to Vietnam, Mattis denied to reporters that an exit was imminent or that he had a rift with Trump, saying he was "on his the president's team".

By Bac Pham & Bennett Murray - Deutsche Presse Agentur - October 16, 20182