US President Donald Trump has warned North Korea "do not underestimate us and do not try us" as he vowed that the United States would defend itself and its allies against Pyongyang's nuclear threat.

Mr Trump issued a stern message to North Korea that Washington "will not be intimidated" as he wrapped up a visit to South Korea with a speech to the National Assembly in Seoul.

He urged countries around the world to join together to isolate Pyongyang by denying it "any form of support, supply or acceptance".

"We will not allow American cities to be threatened with destruction. We will not be intimidated," Mr Trump told South Korean lawmakers.

"And we will not let the worst atrocities in history be repeated here, on this ground we fought and died to secure.

"The world cannot tolerate the menace of a rogue regime that threatens it with nuclear devastation."

Mr Trump returned to tough talk against North Korea a day after he appeared to dial back some of his bellicose rhetoric and instead took more of a carrot-and-stick approach.

He warned Pyongyang on Tuesday of the US military buildup he has ordered in the area but also offered it a diplomatic opening to "make a deal".

'Cruel dictatorship'

Mr Trump also slammed North Korea's "cruel dictatorship" and the severe political oppression inflicted on its people.

He called for global solidarity against the isolated nation, but he also offered leader Kim Jong-Un a possible "path towards a much better future".

"Far from valuing its people as equal citizens, this cruel dictatorship measures them, scores them, and ranks them based on the most arbitrary indication of their allegiance to the state," Mr Trump said in the address.

The North, which this year carried out its sixth nuclear test, generating by far the largest yield to date, has also fired dozens of missiles in recent months.

Two have overflown key US ally Japan and the regime has claimed that it now has the technology to mount a nuclear warhead on a missile that can travel as far as the US mainland.

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'A much better future'

While the speech was heavy on Mr Trump's usual anti-North Korean bombast, he also offered an olive branch, in what he said was a direct message to the country's young leader.

"The weapons you are acquiring are not making you safer," he said.

"They are putting your regime in great danger.

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

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 been in marked contrast 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 Wednesday.

In an article published before his parliamentary address, the paper called the trip "a deliberate scheme aimed at strengthening military threats against us and to light the fuse of nuclear war".

"War maniac Trump is pushing tensions to the extreme by spewing threats of war against us," it said in an editorial.

No DMZ visit for Trump

The speech came after Mr Trump's attempt to make an unannounced visit to the heavily fortified border separating North and South Korea was aborted earlier on Wednesday when dense fog prevented his helicopter from landing, officials said.

Mr Trump tried to travel to the Demilitarised Zone (DMZ) - the doorstep of the North Korean nuclear standoff - on the final day of a 24-hour visit to ally Seoul.

But the US president and his entourage had to turn back when the weather made it impossible for his helicopter to land in the border area, the White House said.

White House spokeswoman Sarah Huckabee Sanders described Trump as disappointed and frustrated at having to abandon the visit.

A visit to the DMZ, despite his aides' earlier insistence he had no plans to go there, would have had the potential to further inflame tensions with North Korea.

Mr Trump's earlier threats to "totally destroy" North Korea if it threatened the United States, and the personal insults he exchanged with North Korean leader Kim Jong Un after the North's most recent missile and nuclear tests, had raised fears in the region of military conflict.