Earlier, US President Donald Trump said in an interview that there was a chance that US Defense Secretary Jim Mattis might step down.

US Defense Secretary Jim Mattis said on Tuesday that he'll remain on Donald Trump's team.

"I'm on his team. We have never talked about me leaving," he speaking to reporters as he flew from Washington to Ho Chi Minh City at the start of Asia trip.

"We just continue doing our job," Mattis added.

He said, 'I'm 100 percent with you'," Mattis quoted Trump as saying during their phone conversation on Monday morning.

When asked whether he was a Democrat or a Republican, Mattis replied that he has never registered for any political party.

The relationship between Trump and Mattis entered the media spotlight after the release of journalist Bob Woodward's book on the Trump administration last month, which portrayed a fictional Mattis privately belittling Donald Trump when speaking to associates.

Jim Mattis denied making any such remarks. He characterized Woodward's book as a "product of a rich imagination."