The US President said Kim Jong Un’s regime “better get their act together”, or they will be in trouble “like few nations have ever been”.
He told Americans that the leader of the rogue state had “disrespected our country greatly” by saying “horrific” things, adding: “And with me, he’s not getting away with it.”
Mr Trump was speaking following a meeting with his top national security advisers, where Pyongyang’s threat to fire missiles towards the US territory of Guamdominated discussion.
He said: “If North Korea does anything in terms of even thinking about an attack on anybody that we love or represent, or our allies or us, they should be very, very nervous,.
“Let’s see what he does with Guam. If he does something in Guam it will be an event the likes of which nobody’s seen before, what will happen in North Korea.”
Mr Trump’s remarks will not assuage the fears of some politicians and foreign policy experts who have called for more measured language, as his vow to unleash “fire and fury” on Pyongyang has unnerved allies in the region.
Earlier, his defence secretary James Mattis had told reporters that the US still prefers a diplomatic approach to easing tensions as a war would be “catastrophic”.
However, when asked whether America was prepared to respond if North Korea commits a hostile act, he replied: “We are ready.”
The President was asked whether he would consider a pre-emptive strike on Pyongyang to thwart its ability to launch a nuclear attack on the US, but he side-stepped the question by saying: “We’ll see what happens.”
China, which is North Korea’s most important ally and trading partner, has faced calls from the White House to “do a lot more” in pressuring Mr Kim to end his country’s ballistic missile programmes.
But an editorial in a state-run newspaper published by the ruling Communist Party has said Beijing is not able to persuade either Washington or Pyongyang to back down.
As well as reiterating calls for calm, the Global Times article said: “China should make clear that if North Korea launches missiles that threaten US soil first and the US retaliates, China will stay neutral.
“If the US and South Korea carry out strikes and try to overthrow the North Korean regime and change the political pattern of the Korean Peninsula, China will prevent them from doing so.”
Pyongyang has not been deterred by Mr Trump’s threats of “fire and fury” – with one state media report describing his warning as a “load of nonsense”.
North Korea has said it will have a fully developed plan for striking Guam with four missiles by mid-August.
According to state media, rockets would land in waters about 18 to 25 miles from the tiny Pacific island, which is home to about 163,000 people as well as a US military base.