Speaking as he flew out for a diplomatic tour that will take him to Vietnam and Singapore, the US Defense Secretary said Washington was not trying to hold back its Pacific rival.

"Obviously, we're not out to contain China," he told reporters. "We'd have taken an altogether different stance had that been considered."

"We're two large powers, or two Pacific powers, two economic powers. There's going to be times we step on each other's toes, so we're going to have to find a way to productively manage our relationship," he said.

But he highlighted key sticking points with Beijing, remarking that the same issues have also rattled some of China's neighbours.

"We remain highly concerned with continued militarisation of features in the South China Sea," Mattis said on a flight to Ho Chi Minh City, the former capital of the US-backed southern regime.

China claims nearly all of the South China Sea -- building a growing roster of military installations on reefs and artificial islands there -- even though Taiwan, the Philippines, Brunei, Malaysia and Vietnam all claim parts of strategic waterway.

Washington regularly conducts freedom of navigation exercises in the sea and earlier this month US defence officials said a Chinese warship sailed within yards of an American destroyer forcing it to change course.

Mattis also singled out what he described as "predatory economic behaviour" by Beijing "where massive debt is piled on countries that fiscal analysis would say they are going to have difficulty, at best, repaying in the smaller countries".

- Escalating tensions -

Sri Lanka famously gave Beijing a 99-year lease to a deep sea port after it was unable to pay back Chinese loans for the $1.4 billion project.

Mattis's visit comes amid escalating tensions between the two superpowers over trade, Beijing's growing military might and allegations by the US that China is interfering in the American electoral process.

His visit to China scheduled for later this month was cancelled because of those tensions.

Trump has levied billions of dollars in tariffs on Chinese imports as he presses Beijing to change restrictive trade practices that he says unfairly hurt American businesses, prompting fears of a shock to the global trade system.

Vietnam is eager to bolster military ties with the United States amid its ongoing dispute with Beijing in the South China Sea, which Hanoi says it has historic rights to.

In March Vietnam hosted a US aircraft carrier in Danang in what analysts said was a clear show of strength and a veiled message to Beijing.

This is Mattis's second trip to Vietnam this year after a visit to Hanoi in January, and the defence chief is slated to go to a former US air base as part of a dioxin remediation program.