TOKYO - US Defence Secretary James Mattis said Friday that the United States stands "100 percent" with Japan on a visit to reassure Asian allies rattled by President Donald Trump's isolationist rhetoric.

Mattis spoke in Tokyo on the first overseas tour by a senior Trump administration official, as concerns rise about the direction of US policy in the region under the protectionist and fiery leader.

He arrived earlier in the day from Seoul, where he warned nuclear-armed North Korea that any attacks by their forces would be met with an "effective and overwhelming" response.

Japan and the US have a decades long security alliance and US-South Korea military ties date back to the 1950-53 Korean War. But on the campaign trail, Trump threatened to withdraw US forces if they do not step up their financial support. Some 28,500 US troops are based in South Korea to defend it against the North, and 47,000 in Japan .

"I want there to be no misunderstanding during the transition in Washington," Mattis told Prime Minister Shinzo Abe, referring to the changeover to Trump from former president Barack Obama.

"We stand firmly, 100 percent, shoulder-to-shoulder with you and the Japanese people."

He added: "I wanted to make certain that Article Five of our mutual defence treaty is understood to be as real to us today as it was a year ago, five years ago and it will be a year and 10 years from now."

Article five commits each country to repel attacks against the other in Japan or territories it administers. The US has said that includes the uninhabited islands in the East China Sea administered by Japan but also claimed by China.

Japanese Foreign Minister Fumio Kishida told reporters after his separate meeting with Mattis that the US defence chief also reconfirmed that commitment.

On the campaign trail, Trump also raised the possibility of Japan and South Korea arming themselves with nuclear weapons, particularly sensitive in Japan - the only country to ever be attacked by atomic bombs - but later drew back from the remarks.

"I do have a hope, and also I am convinced, together with you and President Trump we will be able to demonstrate the unwavering alliance between Japan and the United State both to the public inside Japan as well as outside Japan ," said Abe, who is set to hold a summit with Trump next week in the US .

Abe has repeatedly argued that Japan bears an appropriate share of the costs of the alliance, which he stresses benefits the US , Japan and the broader region.

A senior South Korean defence ministry official told reporters that defence cost sharing did not come up at Mattis ' meeting his South Korean counterpart Han Min-Koo.

A Japanese foreign ministry official also said that Mattis made no mention of base support costs in Tokyo.

Mattis ' visit comes amid heightened fears over North Korea's nuclear ambitions, a major concern for both South Korea and Japan .

Before leaving Seoul, he sought to warn Pyongyang against violent adventurism.

"Any attack on the United States or our allies will be defeated and any use of nuclear weapons would be met with a response that would be effective and overwhelming," Mattis told reporters ahead of a meeting with Han.

North Korea carried out two atomic tests and a series of missile launches last year, and casts a heavy security shadow over the region.

Leader Kim Jong-Un said in his closely watched New Year speech that Pyongyang was in the "final stages" of developing an intercontinental ballistic missile, prompting Trump to tweet: "It won't happen!"

On Thursday Mattis and South Korean Prime Minister Hwang Kyo-Ahn agreed to push through with the deployment of a US missile defence system strongly opposed by China.

The two confirmed that they will go ahead with the installation of the Terminal High Altitude Area Defence system in the South this year as planned.

Beijing fears it will undermine its own ballistic capabilities, weakening its nuclear deterrent. It has repeatedly condemned the move as destabilising regional security, and imposed measures seen as economic retaliation in South Korea.