'Do not try us': Donald Trump warns North Korea against 'fatal' mistake of underestimating US

The president calls the leadership of Kim Jong-un a 'cult'

Donald Trump delivers his speech to South Korea's National Assembly (Reuters)

President Donald Trump has said that North Korea should not make the "fatal" mistake of underestimating the US and its allies by continuing its nuclear programme and extreme rhetoric.

“Today, I hope I speak not only for our countries, but for all civilised nations, when I say to the North: Do not underestimate us. And do not try us,” Mr Trump told South Korea's parliament.

Mr Trump urged all countries to join forces to isolate “the brutal regime of North Korea”, saying the world cannot tolerate “the menace of a rogue regime” that threatens it with nuclear devastation.

In a generally restrained speech - despite the stern warnings - Mr Trump appeared to find a tone that hit home harder than any string of tweets could.

However, he also could not resist a plug for his golf course in Bedminster - mentioning that it held the Women's US Open in a tournament that was won by South Korean golfer Sung Hyun Park.

He called the leadership of Kim Jong-un a "cult" and a "dark fantasy" - saying that the belief Pyongyang could rule the Korean peninsula was "deranged".

“The more successful South Korea becomes, the more decisively you discredit the dark fantasy at the heart of the Kim regime,” Mr Trump said.

“We will not allow American cities to be threatened with destruction. We will not be intimidated,” the president said. “All responsible nations must join forces to isolate the brutal regime of North Korea - to deny it any form of support, supply, or acceptance.”

Mr Trump said Washington “will not be intimidated” as he wrapped up a visit to South Korea with a speech to the National Assembly in Seoul.

“We will not allow American cities to be threatened with destruction. We will not be intimidated,” he 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.”

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.

But he did offer a "path" that Mr Kim and the regime in Pyongyang. “The weapons you are acquiring are not making you safer," he told the North Korean leader. "They are putting your regime in grave danger. Every step you take down this dark path increases the peril you face.”

“Despite every crime you have committed before God and man, we are ready to offer...a path to a much better future,” Mr Trump added.

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”.

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 in the day 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. He was then due to fly to China, where US officials say he will press a reluctant President Xi Jinping to tighten the screws further on Pyongyang.

However, Mr Trump 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 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.

As the US president looked towards the visit to Beijing, he called on China and Russia in particular to ensure they impose all financial and diplomatic sanctions they can on the regime in Pyongyang.

Reuters contributed to this report