Any nuclear attack by North Korea would trigger an "effective and overwhelming" response, US Defence Secretary James Mattis said Friday as he sought to reassure Asian allies rattled by President Donald Trump's isolationist rhetoric.

Mattis spoke in the South Korean capital of Seoul 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 in Tokyo later in the day for a meeting with Japanese Prime Minister Shinzo Abe and is set to hold talks with Japanese defence minister Tomomi Inada on Saturday.

South Korea has enjoyed US security protection since the 1950-53 Korean War, but on the campaign trail, Trump threatened to withdraw US forces from it and Japan if they do not step up their financial support.

Some 28,500 US troops are based in South Korea to defend it against the nuclear-armed North, and 47,000 in Japan.

Pyongyang was continuing to "engage in threatening rhetoric and behaviour", said Mattis, who first came to the South in 1972 as a 21-year-old lieutenant in the US military.

"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 his South Korean counterpart Han Min-Koo.

He was in Seoul to "underscore America's priority commitment to our bilateral alliance" and make clear the administration's "full commitment" to defending South Korea's democracy," he said.

Han added that the alliance "reaffirms its firm will and strength to remain unwavering against all challenges and adversaries".

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!"

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Ahead of his departure for Japan, Mattis laid a wreath at the Tomb of the Unknown Soldier in the Seoul National Cemetery, where he met several hundred supporters and Korean War veterans waving American flags and pictures of Trump.

- 'Top priority' -

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 Defense (THAAD) 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.

The dispute makes it harder to convince Beijing -- the North's most important diplomatic protector and main source of aid and trade -- to act against its neighbour, analysts say.

"Deepening tensions between China and the US adds to the North's strategic value in the eyes of China," Lee Ji-Yong, a professor at South Korea's government-financed Institute for Foreign Affairs and Security told AFP.

"It will make more difficult for the US to persuade China to cooperate in pressuring the North to give up its nuclear arsenal."

Mattis' visits to South Korea and Japan, he added, were "a message that the Trump administration is giving top priority to ensuring security on the Korean peninsula against North Korea's nuclear sabre-rattling and the US is a reliable security partner in the region".

Japan's Abe -- who is scheduled to meet Trump next week in Washington -- told lawmakers he intends to press Mattis about "the significance of the Japan-US alliance".

Mattis' tour comes as relations between the US and countries such as Mexico and Australia get off to a rocky start.

The Washington Post reported late Wednesday that Trump ripped into his Australian counterpart Malcolm Turnbull during a call last week, with the US president apparently fuming at a refugee accord he called "dumb" and cutting the conversation short.

Australia is a close US ally, and one of the so-called "Five Eyes" countries with which the US routinely shares sensitive intelligence.

Trump has meanwhile angered Mexicans by ordering the construction of a massive border wall and vowing to make their country pay for it.