North Korea must "make a deal" on abandoning its nuclear weapons, US President Donald Trump said, taking his most conciliatory tone yet with Pyongyang after months of inflammatory threats.

While Trump's comments on Tuesday from Seoul, South Korea, appeared to be the first offer by the US president for Pyongyang's leadership to sit down and negotiate, he also highlighted the military build-up in the Asia-Pacific region and his plan to use it "if need be".

Those assets include three aircraft carrier strike groups meeting for war manoeuvres later this month, as well as a nuclear submarine that is already in position.

"It really makes sense for North Korea to come to the table and make a deal that is good for the people of North Korea and the people of the world," Trump told reporters at a joint news conference with South Korean President Moon Jae-in.

"We are showing great strength and I think they [North Korea] understand we have unparalleled strength … We have many things happening that we hope to God that we never have to use."

Mocking leader Kim Jong-un as "little rocket man", Trump has previously warned of "fire and fury" and said the US would "totally destroy" North Korea if it launched an attack on the United States or its allies.

Pyongyang responded by threatening to carry out its first atmospheric nuclear test - after six conducted underground - with Kim describing Trump as a "mentally deranged US dotard".

President Moon - speaking at a dinner later on Tuesday with Trump - said the US' military might acts as a deterrent against an attack by North Korea.

Aside from a nuclear strike, Pyongyang has hundreds of heavy artillery weapons trained on Seoul - a city of about 11 million people. The US also has some 28,000 American troops in South Korea, a legacy of the 1950-53 war.

Analysts predict millions of people would die in the first few days if war breaks out.

"There must never be another war on the Korean Peninsula. In that sense, the United States is being a great help to us," Moon said, according to Yonhap news agency.

"Our overwhelming superiority of power based on the Korea-US alliance will eventually make North Korea stop its reckless provocations and come to the path of denuclearisation."

Citing countries without nuclear weapons that suffered "invasion and plunder" by the United States, North Korea has said it will never abandon its nuclear weapons.

'Threatening millions'

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Trump and Moon also announced that South Korea would buy "billions" of dollars worth of state-of-the-art American military hardware, possibly including nuclear-powered submarines that can intercept North Korea's ballistic missiles.

"North Korea's sixth test of a nuclear device and its missile launches are a threat not only to the people of South Korea, but to the people all across our globe," said Trump.

"We will together confront North Korea's actions and prevent the North Korean dictator from threatening millions of innocent lives … The US stands prepared to defend itself and its allies using the full range of our unmatched military capabilities - if need be."