US President Donald Trump issued a rare - if tepid - defence of Joe Biden on Sunday, saying Pyongyang's depiction of him as "a rabid dog" who should be "beaten to death" went a bit too far.

"Joe Biden may be Sleepy and Very Slow," Trump tweeted, using his nicknames for the former vice president and potential Democratic presidential candidate, "but he is not a 'rabid dog'. He is actually somewhat better than that."

The president was responding to a conservative commentator's tweet about a particularly visceral attack on Biden issued on Friday by North Korea.

The North's official KCNA news agency said Biden had shown "the temerity to dare slander the dignity of the supreme leadership" of North Korea.

"Rabid dogs like Biden can hurt lots of people if they are allowed to run about," the KCNA statement said. "They must be beaten to death with a stick."

It was not clear what had provoked Pyongyang's ire. But the angry response came after the Biden campaign released an ad condemning Trump's foreign policy and saying that "dictators and tyrants are praised, our allies pushed aside."

The word "tyrants" was heard at the exact moment a picture appeared on the screen of Trump and North Korean leader Kim Jong-Un shaking hands at their Singapore summit last year.

It was not clear to what extent Trump was being tongue-in-cheek in saying Biden - a favourite target of the president's - was "somewhat better than that", nor whether the president was perhaps using his tweet as a way to re-engage with Kim.

"I am the only one who can get you where you have to be," Trump's tweet said, referring to Kim. "You should act quickly, get the deal done. See you soon!"

Talks between the two sides on a deal for North Korea's denuclearisation stalled after a Hanoi summit between Kim and Trump broke up in February. Working-level talks in Sweden last month also broke down.

But Pyongyang welcomed news on Sunday that joint US-South Korean air drills are being postponed in what US Defence Secretary Mark Esper called an "act of goodwill" toward the nuclear-armed North.