President Donald Trump issued an apocalyptic warning to North Korea on Tuesday, saying it faces "fire and fury" over its missile program, after US media reported Pyongyang has successfully miniaturized a nuclear warhead.
North Korea raised the stakes just hours later, saying it was considering missile strikes near US strategic military installations on the Pacific island of Guam.
Once finalised, the plan could be put into action at "any moment" once leader Kim Jong-Un made a decision, the official Korean Central News Agency (KCNA) quoted a military statement as saying.
Trump's remarks marked a sharp rise in rhetoric from the US - and appear to echo Pyongyang's own regular threats, most recently repeated on Monday, to turn Seoul into a "sea of flames".
"North Korea best not make any more threats to the United States," said Trump, speaking from his golf club in New Jersey. "They will be met with fire and fury like the world has never seen."
They also represent a change in tone from Secretary of State Rex Tillerson's assurances last week that Washington was not seeking regime change in Pyongyang.
The remote island of Guam - a 210-square-mile dot in the Pacific - is a key US military outpost and home to some 6,000 US troops spread across facilities including the sprawling Anderson Air Force Base, as well as Naval Base Guam.
The North's statement came after US B1-B bombers overflew the Korean peninsula on Tuesday, which KCNA said "proves that the US imperialists are nuclear war maniacs".
The Washington Post quoted a Defense Intelligence Agency analysis as saying officials think North Korea now has "nuclear weapons for ballistic missile delivery" - including in its intercontinental ballistic missiles (ICBMs) - making it a potent threat against neighbors and possibly the United States.
The Pentagon did not comment on the story, but the Post said two US officials familiar with the analysis had verified the assessment's broad conclusions, and CNN said it had confirmed the report.
Experts have long differed over the North's exact capabilities, and a similar DIA assessement four years ago was dismissed by other intelligence organisations.
But all agree it has made rapid progress under Kim Jong-Un.
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.
Trump said Kim "has been very threatening beyond a normal state."
"As I said, they will be met with the fire and fury and, frankly, power," he told reporters.
US officials have repeatedly said this year that military action against the North was an "option on the table."
But analysts and politicians reacted to the US president's latest remarks with derision.
"Trying to out-threaten North Korea is like trying to out-pray the Pope," John Delury of Seoul's Yonsei University said on Twitter.
Security commentator Ankit Panda added: "Trump's comments were dangerous and unusual; North Korea's threat was also specific, but not unusual."
Congressman Eliot Engel, the Democratic senior member of the House Committee on Foreign Affairs, chastised Trump for drawing an "absurd" red line that Kim would inevitably cross.
"North Korea is a real threat, but the president's unhinged reaction suggests he might consider using American nuclear weapons in response to a nasty comment from a North Korean despot," Engel said in a statement.
Deputy Secretary of State John Sullivan said Washington continues to work to make sure China and other countries enforce tough new United Nations sanctions passed at the weekend - the seventh set imposed on Pyongyang over its weapons ambitions.
The Post also reported that another intelligence assessment estimated North Korea now has up to 60 nuclear weapons, more than previously thought.
Despite the advance, North Korea still must overcome technical hurdles before it will be seen to have perfected the technology.
After Kim's second ICBM test, experts said it appeared the "re-entry vehicle" that would carry a warhead back into Earth's atmosphere from space had failed in the intense heat.
Former Los Alamos National Laboratory director Siegfried Hecker told the Bulletin of the Atomic Scientists he did not think North Korea yet had sufficient missile or nuclear test experience "to field a nuclear warhead that is sufficiently small, light and robust to survive an ICBM delivery."
North Korea has vowed that the new UN sanctions would not stop it from developing its nuclear arsenal, and that it would never negotiate it away.