The longtime Washington adage that politics should stop at the water's edge took a new beating over the weekend, as US President Donald Trump cited the North Korean dictator as agreeing with his own dim view of Democrat Joe Biden.

A White House spokeswoman has brushed off criticism of president Trump's remark and confirmed the two leaders' critical opinion of the former US vice president.

Donald Trump had tweeted, at the outset of an official visit to Japan, that he "smiled when (Kim) called Swampman Joe Bidan a low IQ individual, & worse."

Mr Trump later corrected the spelling of Biden's name, but his reliance on a dictator's opinion to ambush a political foe from overseas drew sharp criticism from Joe Biden's team and others.

"I would say the tweet speaks for itself, but it's so unhinged and erratic that I'm not sure anyone could even say that with a straight face," an unnamed Biden aide told ABC News.

During a satellite interview from Tokyo with NBC, White House spokeswoman Sarah Sanders was asked "why Americans should not be concerned that the president of the United States is essentially siding with a murderous authoritarian dictator over a former vice president of the United States."

"The president's not siding with that," Ms Sanders replied, "but I think they agree in their assessment of former vice president Joe Biden."

"The president doesn't need somebody else to give him an assessment of Joe Biden."

Mr Biden, a six-term senator before serving as No. 2 to President Barack Obama, has taken a substantial lead in early polling on the nearly two dozen Democrats seeking the 2020 presidential nomination.

He has increasingly been a favored target of Donald Trump, who refers to the Democrat as "Sleepy Joe."