A day after North Korea declared it would make the United States pay dearly for fresh sanctions on the country, US President Donald Trump has answered in kind.
Mr Trump has added further animosity to an already tense and hostile relationship with a response just as fiery for North Korean leader Kim Jong-un.
"North Korea best not make any more threats to the United States. They will be met with fire and fury like the world has never seen. He has been very threatening, beyond a normal statement. And as I said, they will be met with fire, fury and, frankly, power, the likes of which this world has never seen before."
US stocks fell in a late-afternoon selling spree following President Trump's comments.
The Dow Jones and Nasdaq both dropped.
But White House counsellor Kellyanne Conway has refused to be drawn into the matter.
She was asked if she could clarify Mr Trump's comments during a press conference about drug addiction with health secretary Tom Price.
"No, I can't. I think the President's comments were very strong and obvious. I know all of you covered them live. And I would defer to other members of Dr Price's cabinet to comment further."
CNN television commentators in the United States have suggested his remarks give support to a longstanding North Korean narrative that the United States is preparing for war.
North Korea says that is why it must accelerate its missile and nuclear programs.
President Trump's comments also conflict with the message of his secretary of state a day earlier.
Rex Tillerson had offered a more conciliatory approach, saying the United States and North Korea could, eventually, improve relations.
"Well, the best signal that North Korea could give us that they're prepared to talk would be to stop these missile launches. You know, we've not had an extended period of time where they have not taken some type of provocative action by launching ballistic missiles. So, I think that would be the first and strongest signal they could send us is just stop ... stop these missile launches. Obviously, we have other means of communication open to them, and to certainly hear from them, if they have a desire to want to talk."
In what would be a stunning development, The Washington Post reports US analysts believe North Korea has produced a miniaturised nuclear warhead that can fit inside its missiles.
The newspaper reports the new analysis was completed last month.
The BBC's senior political correspondent, Jane O'Brien, says if confirmed, it would bring North Korea closer to becoming a fully fledged nuclear power.
"If confirmed -- and we haven't had that confirmation from the Pentagon yet -- then, yes, it is a significant development but not unexpected. I think what's concerning experts at the moment is the speed at which they've been able to develop a warhead, a nuclear warhead, that is small enough to be able to fit into one of these intercontinental ballistic missiles that they have successfully tested. Now that would mean that the US, for the first time, is under direct threat from a nuclear strike by North Korea. However, it has to be said that there are other technical issues the North Koreans have to surmount, and that is getting a nuclear warhead that would not be destroyed on re-entry into the atmosphere. So, the threat is growing, but it's not there yet."
Over the weekend, the United Nations Security Council unanimously adopted new sanctions on North Korea.
The country's exports of coal, iron and seafood will be banned, potentially cutting its $3 billion annual export revenue by one-third.
The United Nations is pressuring the North Koreans to end their nuclear program.
But in a statement, North Korea's foreign minister said the United States would pay dearly for the sanctions.
He also insisted his country has no intention of using nuclear weapons against any other country except the United States.
