President Donald Trump has warned North Korea "things will happen to them like they never thought possible" if it does anything to the United States.
He said the regime would be in trouble "like few nations have ever been" if they do not "get their act together".
His comments came after Pyongyang announced it had a plan to fire four missiles near the US territory of Guam.
The New Zealand Defence Force has confirmed an Air Force aircraft is in Guam as part of a military exercise.
Tensions between the two countries have escalated in recent weeks after North Korea tested two intercontinental ballistic missiles in July.
The UN recently approved further economic sanctions against Pyongyang as a result of its nuclear weapons programme.
Speaking on Thursday (Friday NZT) at his golf club in Bedminster, New Jersey, Mr Trump offered an olive branch, saying the US would always consider negotiations.
But Republican president said his own statements had not been tough enough on Pyongyang, despite his threat this week to rain "fire and fury" upon the regime.
Pyongyang earlier dismissed the US president's dire warnings as "nonsense".
Mr Trump doubled down in his latest comments, saying: "It's about time someone stood up for the people of our country."
He railed against previous US administrations for being too weak on North Korea, adding that the tiny dictatorship's pursuit of nuclear weapons was a "tragedy".
He also chided China, saying they could do "a lot more" to intervene on the Korean peninsula.
When asked if there was a possibility of a pre-emptive strike against Pyongyang, Mr Trump said: "We don't talk about that. We never do."
"I will tell you this, if North Korea does anything in terms of even thinking about attack of anybody that we love or we represent or our allies or us they can be very, very nervous.
"I'll tell you why ... because things will happen to them like they never thought possible.
"I will tell you this, North Korea better get their act together or they're gonna be in trouble like few nations have ever been in trouble in this world."
North Korea said on Wednesday (Thursday NZT) it planned to fire medium-to-long-range rockets towards Guam, where US strategic bombers are based.
However, there has been no indication that any attack on Guam by North Korea is imminent.
In a message to the public, Guam governor Eddie Baza Calvo said there was currently "no threat" to the island and the Marianas archipelago, but that Guam was "prepared for any eventuality".
Mr Trump meanwhile denied there were any mixed messages from his administration.
Earlier this week, US Secretary of State Rex Tillerson played down the rhetoric between the two sides.
But in an interview with the BBC on Thursday, White House Deputy Assistant Sebastian Gorka dismissed the top diplomat's comments.
"You should listen to the president," he said.
"The idea that Secretary Tillerson is going to discuss military matters is simply nonsensical.
"It is the job of Secretary Mattis as Secretary of Defense to talk about the military options and he has done so unequivocally today."
US Secretary of Defense Jim Mattis said on Wednesday Pyongyang would be "grossly overmatched" in any war against the US and its allies.
- BBC / Reuters