US President Donald Trump and First Lady Melania arrive to lay a wreath at the National Cemetery in Seoul. Picture: Kim Hong-Ji/AFP

Seoul - US President Donald Trump issued a stark warning to North Korean leader Kim Jong Un on Wednesday, telling him that the nuclear weapons he is developing "are not making you safer, they are putting your regime in grave danger".

"Do not underestimate us and do not try us," Trump told North Korea as he wrapped up a visit to South Korea with a speech to the National Assembly in Seoul.

Trump used some of his toughest language yet against North Korea in a wide-ranging address that lodged specific accusations of chilling human rights abuses against Pyongyang. He called on countries around the world to isolate Pyongyang by denying it "any form of support, supply or acceptance".

Trump painted a dystopian picture of North Korea, saying people were suffering in "gulags" and some bribed government officials to work as "slaves" overseas rather than live under the government at home. He offered no evidence to support those accusations.

Trump's return to harsh, uncompromising language against North Korea came a day after he appeared to dial back the bellicose rhetoric that had fuelled fears across east Asia of the risk of military conflict. On Tuesday, Trump had even offered a diplomatic opening to Pyongyang to "make a deal".

He went mostly on the attack in Wednesday's speech but did promise a "path to a much better future" for North Korea if it stopped developing ballistic missiles and agreed to "complete, verifiable and total denuclearisation" – something Pyongyang has vowed never to do.

"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."

"The world cannot tolerate the menace of a rogue regime that threatens it with nuclear devastation," Trump said, speaking as three U.S. aircraft carrier groups sailed to the Western Pacific for exercises - a rare show of such US naval force in the region.

'GRAVE DANGER'

Aiming his words directly at Kim, Trump insisted: "The weapons that you are acquiring are not making you safer, they are putting your regime in grave danger. Every step you take down this dark path increases the peril you face."

However Trump, whose strategy has stressed sanctions and military pressure instead of diplomacy, did not spell out any new approach to force North Korea to abandon its missile and weapons programmes.

North Korea has made clear it has little interest in negotiations at least until it develops a nuclear-tipped missile capable of hitting the U.S. mainland, something US intelligence officials say it may be just months away from achieving.

"North Korea is a country ruled by a cult," Trump said in a speech that was interrupted several times by applause and ended with a standing ovation.

He stopped short, however, of repeating the derisive nickname "little Rocket Man" that he has used to describe the young North Korean leader.

Kim, for his part, has called Trump "mentally deranged".

The speech came after 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.

Trump tried to travel to the Demilitarized Zone (DMZ) - the doorstep of the North Korean nuclear standoff – near the end of a 24-hour visit to ally Seoul. He was then due to fly to China, where U.S. officials say he will press a reluctant President Xi Jinping to tighten the screws further on Pyongyang.

However, 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 "frustrated" at having to abandon the trip.

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.

Trump's earlier threats to "totally destroy" North Korea if it threatened the United States, and the personal insults he exchanged with Kim after the North's most recent missile and nuclear tests, had raised fears in the region of a potential spiral into war.

CHINA'S ROLE

Just before departing for Beijing, Trump specifically cited China, North Korea's main trading partner, as one of the countries that must fully enforce international sanctions against Pyongyang and downgrade diplomatic and commercial 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 said.

Trump will try to convince Xi to squeeze North Korea further with steps such as limits on oil exports, coal imports and financial transactions.

But it is far from clear if Xi, who has just consolidated his power at a Communist Party congress, will agree to do more.

China has repeatedly said its leverage over Pyongyang is exaggerated by the West and that it is already doing all it can to enforce sanctions.

Despite that, Xi may be mindful that Trump has held off on trade actions against China that he loudly threatened during the 2016 presidential campaign to give Beijing more time to make progress on North Korea.