The war of words between the two countries has escalated. Markets have fallen on the emboldened rhetoric that came from President Trump, who promised to meet any challenge presented by North Korea.
The United States on Wednesday warned North Korea that the pursuit of its nuclear drive could lead to the collapse of Kim Jong-Un's regime, as President Donald Trump brandished America's nuclear might as a deterrent.
The further escalation in Washington's war of words with Pyongyang came a day after Trump stunned the world with a bold-faced message to Kim, saying his country faced "fire and fury like the world has never seen."
Those comments triggered expressions of concern from China as well as from US allies, and stock markets and the dollar have slipped as investors seek safe-haven investments.
Defense Secretary Jim Mattis, who has often emphasised the devastating costs any conflict with North Korea could have, delivered a statement that seemed to back up his boss's bellicose language.
He said North Korea must stop isolating itself and "stand down" in its pursuit of nuclear weapons, and Pyongyang "should cease any consideration of actions that would lead to the end of its regime and the destruction of its people."
The Pentagon chief also underscored the reality of North Korea's nascent missile capabilities, saying they are "grossly overmatched" by the US and that Pyongyang would lose any arms race or conflict it started.
Trump earlier had boasted on Twitter that America's nuclear arsenal was "far stronger and more powerful than ever before," after North Korea said it was considering a missile strike near the tiny US Pacific territory of Guam.
"Hopefully we will never have to use this power, but there will never be a time that we are not the most powerful nation in the world!" Trump tweeted.
Response was known
Amid reports that Trump's comments have taken his inner circle by surprise, White House spokeswoman Sarah Huckabee Sanders said the National Security Council and other officials knew the "president was going to respond... with a strong message in no uncertain terms."
Still, Trump's tone was at odds with that of Secretary of State Rex Tillerson, who said he did not believe "there is any imminent threat" to Guam or other US targets, and hoped that diplomatic pressure would prevail in the crisis.
"I think Americans should sleep well at night, have no concerns about this particular rhetoric of the last few days," Tillerson said.
UN Secretary-General Antonio Guterres said he was deeply concerned about the tensions on the Korean Peninsula and was "troubled" by the increase in confrontational rhetoric.
"The Secretary-General welcomes all initiatives that will help to de-escalate tensions and result in a return to diplomacy," his spokesman Stephane Dujarric said.
Trump's language has become increasingly hard-edged since Pyongyang carried out a pair of successful intercontinental ballistic missile tests (ICBM) in July, which put the US mainland in range for the first time.
Attack on Guam
Pyongyang's KCNA state news agency reported Wednesday that plans were being drawn up for missile strikes against Guam that could be put into action at "any moment" after Kim gave the order.
Tillerson, who refueled in Guam while flying home from Southeast Asia, said Trump was determined to send an unequivocal message to Kim.
"What the president is doing is sending a strong message to North Korea in language that Kim Jong-Un would understand, because he doesn't seem to understand diplomatic language," he said.
The island's governor Eddie Calvo reassured residents there was currently "no threat" to the territory, which has a total population of more than 160,000 and houses two US military installations.
In the capital Hagatna, islanders kept their cool.
"It's not like there's anything we can do anyway. This is a small island. There's nowhere to run to," resident James Cruz said.
Experts have long differed over the North's exact capabilities but all agree it has made rapid progress under Kim.
Last month, Pyongyang carried out its first two successful ICBM launches, the first -- described by Kim as a gift to "American bastards" -- showing it could reach Alaska, and the second extending its range even further, with some experts suggesting New York could be vulnerable.
On Tuesday, The Washington Post quoted a US Defense Intelligence Agency analysis as saying officials think North Korea now has "nuclear weapons for ballistic missile delivery" -- including by its ICBMs -- making it a potent threat against neighbors and possibly the United States.
The North's current ability to launch an accurate nuclear strike remains open to question, with analysts suggesting it has yet to overcome major technical hurdles.
US officials have repeatedly said military action against the North was an "option on the table," though Tillerson said nothing had "dramatically changed" in the military equation in the past 24 hours.
The UN Security Council unanimously approved a series of sanctions over the weekend which could cost North Korea $1 billion a year, with even the regime's main ally China voting for the US-drafted proposal.
While the sanctions vote was seen as a diplomatic triumph for the US, there is widespread unease about the ramping up in tensions.
The North's main ally China warned against "words and actions" that would stoke tensions, while Germany said it was watching the "increasing rhetorical escalation" with concern.
France meanwhile praised Trump's "determination" in standing up to Pyongyang.