President Donald Trump warned the "cruel dictatorship" of North Korea against underestimating the United States on Wednesday, but offered leader Kim Jong-Un a better alternative if he gives up his nuclear ambitions.

In an address to the South Korean parliament — the first by a US president for 24 years — Trump painted a dark picture of Pyongyang as an oppressive, despotic regime.

He called on the world to act, specifically singling out the North's allies China — where he was headed for later on Wednesday — and Russia.

"North Korea is a country ruled as a cult," the US leader declared, a year to the day after his election victory.

"At the centre of this military cult is a deranged belief in the leader's destiny to rule as parent protector over a conquered Korean peninsula and an enslaved Korean people."

South Korean lawmakers applauded as the US president, whose tour of Asia has been dominated by fears over the nuclear-armed North, vowed not to be intimidated and warned Pyongyang that it should not test American resolve.

The North carried out its sixth nuclear test in September, by far its most powerful to date, and has fired dozens of missiles in recent months.

Two have overflown key US ally Japan, and Pyongyang says it can mount a nuclear warhead on a rocket with the US mainland within range.

Trump, who previously vowed a North Korean intercontinental ballistic missile (ICBM) capability "won't happen", again warned Pyongyang not to strike.

"We will not permit America or our allies to be blackmailed or attacked," he said, or "allow American cities to be threatened with destruction".

Ahead of his meeting with Chinese President Xi Jinping, Trump called on the world to unite against the threat from North Korea.

"You cannot support, you cannot supply, you cannot accept," he said, urging China and Russia to fully implement UN sanctions, downgrade diplomatic ties, and sever all trade and technology ties.

"To those nations that choose to ignore this threat, or, worse still, to enable it, the weight of this crisis is on your conscience," he added ahead of his trip to China which is responsible for about 90 percent of the North's commerce.

• DMZ no-go -

Earlier, Trump was forced to abandon a surprise visit to the Demilitarized Zone (DMZ) that divides the two Koreas because of bad weather, leaving him "pretty frustrated" according to the White House.

South Korean President Moon Jae-In, who had flown earlier before fog closed in, was left waiting for him at a guard post on the border, which bristles with electric fences, minefields and anti-tank barriers.

In his speech, Trump described the DMZ as "the line that today divides the oppressed and the free", where "the flourishing ends, and the prison state of North Korea sadly begins".

The North's ruling Kim dynasty has led the impoverished, isolated country with an iron fist and pervasive personality cult for three generations, showing no tolerance for political dissent.

The regime has for decades been criticised for a range of rights abuses including torture, rape and execution of perceived critics or those trying to flee the country.

It is also known to operate prison camps where hundreds of thousands languish under forced labour, and its 25 million people are barred from contact with the outside world such as foreign television or internet access.

While Trump condemned the authorities, he also made overtures to leader Kim Jong-Un, who has overseen rapid advances in its weapons technology.

In what he said was a direct message to the country's young leader, he told him: "The weapons you are acquiring are not making you safer. They are putting your regime in great danger.

"North Korea is not the paradise your grandfather envisioned," he went on. "It is a hell that no person deserves."

"Yet despite every crime you have committed against God and man, we will offer a path towards a much better future."

It would have to begin, though, with Pyongyang stopping its ballistic missile development, Trump said, and "complete verifiable and total denuclearisation".

The US leader's tone in Seoul, just an hour from the DMZ and a city whose 10 million inhabitants would find themselves on the frontline of any conflict, has contrasted to his previous warnings of "fire and fury".

"Ultimately, it will all work out," he said on Tuesday. "It always works out. It has to work out."

Even so the North's Rodong Sinmun newspaper, the mouthpiece of the ruling Workers' Party, condemned his visit in an editorial Wednesday, calling it "a deliberate scheme aimed at strengthening military threats against us and to light the fuse of nuclear war".