President Donald Trump warned the "cruel dictatorship" of North Korea against underestimating the United States Wednesday, but offered leader Kim Jong-Un a better future 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 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."
Analysts said the criticisms could provoke a reaction from the North.
But 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 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.
"We will not permit America or our allies to be blackmailed or attacked," Trump said, or "allow American cities to be threatened with destruction".
Ahead of his meeting with Chinese President Xi Jinping -- whose country is responsible for about 90 percent of the North's commerce -- Trump called on the world to unite against the threat from Pyongyang.
"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.