North Korea has threatened to resume referring to US president Donald Trump as a "dotard", raising the prospect of a return to a war of words with a negotiating deadline approaching.
Pyongyang has set Washington an end-of-year time limit to offer it new concessions in deadlocked nuclear negotiations, and has said it will adopt an unspecified "new way" if nothing acceptable is forthcoming.
Trump and North Korean leader Kim Jong Un -- whose countries and their allies fought each other to a standstill in the 1950-53 Korean War -- engaged in mutual insults and threats of devastation in 2017, sending tensions soaring before a diplomatic rapprochement the following year.
But denuclearisation negotiations have been at a standstill since a summit in Hanoi broke up in February.
Trump on Tuesday indicated military action was still possible when he was asked about Pyongyang on the sidelines of a NATO summit in Britain.
"He definitely likes sending rockets up, doesn't he? That's why I call him 'Rocket Man'," Trump said, reprising one of his previously favoured nicknames for Kim.
Pyongyang reacted stiffly late Thursday, with vice foreign minister Choe Son Hui saying the comments were made with "no courtesy when referring to the supreme leadership... of the DPRK".
"If this is meant to make expressions, reminiscent of those days just two years ago when a war of words was fought across the ocean... it will be a very dangerous challenge," she said in a statement carried by the North's state news agency KCNA.
Referring to Trump by name and reprising Kim's own previously preferred insult, she said that any repetition "must really be diagnosed as the relapse of the dotage of a dotard".
North Korea's comments came a day after it warned that if the US used military force against the North it would take "prompt corresponding actions at any level" in the event of military action by Washington.
At the Nato summit, Trump said: "We have the most powerful military we've ever had, and we're by far the most powerful country in the world."
"Hopefully, we don't have to use it, but if we do, we'll use it. If we have to, we'll do it," he added.
KCNA, known for its flamboyant and sometimes antiquated language, has previously labelled former US President George W. Bush a "half-baked man", ex-South Korean leader Park Geun-hye a "crafty prostitute", and called former US leader Barack Obama her "pimp".
The North has issued a series of increasingly assertive comments in recent weeks as its negotiating time limit approaches. Kim's New Year speech, a key political set-piece in the isolated country, is also due on January 1.
On Wednesday another Pyongyang official said, in response to Trump's comments made Tuesday, the use of armed forces is "not the privilege of the US only".