Donald Trump and Kim Jong-un “agree in their assessment” that former vice-president and 2020 Democratic candidate Joe Biden has a “low IQ”, White House press secretary Sarah Sanders said.

Speaking from Tokyo, where the president is on a state visit, Sanders told NBC’s Meet the Press Trump’s point of view was not guided by the North Korean dictator, whose state media this week attacked Biden as a “fool of low IQ”.

In a tweet on Saturday night, Trump called the former vice-president a “low IQ individual”. He first misspelled Biden’s name as “Bidan”.

“The president doesn’t need somebody else to give him an assessment of Joe Biden,” Sanders said. “He’s given his own assessment a number of times. I think you’ve seen it. I’m sure you’ve covered it on your programme. The president watched him and his administration with President Obama fail for eight years.”

Sanders also rejected the contention that Americans should be “concerned that the president of United States is essentially siding with a murderous authoritarian dictator over a former vice-president in the United States”.

“The president’s not siding with that,” she said. “But I think they agree in their assessment of former vice-president Joe Biden.”

Trump caused familiar diplomatic consternation with his tweet, in which he also brushed off recent North Korean missile launches that were condemned by the UN and his own national security adviser, John Bolton.

Bolton on Saturday told reporters he had “no doubt” the tests violated UN resolutions. A number of disagreements between Bolton and Trump have come to light, including over Venezuela and Iran.

Bolton’s comments on North Korea were nonetheless the first time a senior administration official has confirmed that the short-range ballistic missiles are in contravention of UN resolutions. One flew nearly 300 miles, making it a direct threat to South Korea, a US ally, if not to Japan.

In the midst of Trump’s visit to Tokyo, the president appeared to place confidence in his relationship with Kim ahead of US intelligence assessments.

“North Korea fired off some small weapons,” Trump tweeted, “which disturbed some of my people, and others, but not me. I have confidence that Chairman Kim will keep his promise to me [and] also smiled when he called Swampman Joe Biden a low IQ individual [and] worse. Perhaps that’s sending me a signal?”

A Biden campaign aide responded: “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.”

Trump has expressed belief that Kim may be sending him secret messages comes as the administration continues to defend the results of two high-profile summits aimed at convincing North Korea to abandon its nuclear weapons program.

Asked if the missile tests were “bothering” Trump, Sanders said: “The president’s focus in all of this process is on continuing the very good relationship that he has with Chairman Kim. And he feels good that the chairman will stay firm with the commitment that he made to the president and move towards denuclearization.”

Sanders continued: “The president still feels comfortable and confident in the relationship that he has with Chairman Kim and that he’s going to stay true to the commitment that he made to the president.”

Sanders said it was “not true to say nothing’s come of” the summits, which were held in Singapore in June 2018 and Hanoi in February this year.

“There have been steps,” she said, adding: “For a significant period of time there was no missile testing. We got hostages back home to the United States and remains of American war heroes. I don’t know how you can say that that’s nothing.”