Donald Trump has dismissed concerns about North Korea’s recent missile tests, calling them “small weapons”, a day after his national security adviser said there was no doubt the launches violated UN security council resolutions.

The US president tweeted on Sunday: “North Korea fired off some small weapons, which disturbed some of my people, and others, but not me. I have confidence that Chairman Kim will keep his promise to me”.

His adviser John Bolton said on Saturday UN resolutions prohibited the launch of any ballistic missiles and urged Kim Jong-un to return to denuclearisation talks.

Despite scant progress from two summits with the North Korean leader, Trump has made much of his personal relationship with Kim, saying last year that he “fell in love” with the dictator after receiving “beautiful letters” from him.

After Pyongyang conducted two tests of short-range missiles in the space of five days this month, Trump said he did not think North Korea was “ready to negotiate”. “We’re looking at it very seriously right now,” Trump told reporters in the White House at the time. “Nobody’s happy about it.”

US officials had previously expressed optimism that Trump would meet Kim for a third summit, aimed at persuading North Korea to dismantle its nuclear programme in return for sanctions relief.

In Sunday’s tweet, Trump called Joe Biden “Swampman Joe Bidan [sic]” before correcting it and applauded North Korea for calling the Democratic presidential hopeful a “fool of low IQ” earlier in the week. Trump wondered if Kim was sending him “a signal”. Biden had earlier accused the US president of courting “dictators and tyrants” such as Kim and Russian president Vladimir Putin.

Trump is in Japan on a state visit and is expected to discuss North Korea’s nuclear programme with regional ally and prime minister Shinzo Abe, probably at least in part during a game of golf. The president has said he wants a deal to address the trade imbalance between the two countries and ensure fairness and reciprocity.

“Japan has had a substantial edge for many, many years, but that’s OK,” he said. “Maybe that’s why you like us so much.”

Trump is also due to become the first foreign leader to be received by Emperor Naruhito. Naruhito and Empress Masako, who studied at Harvard, will host an elaborate dinner on Monday night. In Washington on Thursday, Trump made clear that he was flattered by the invitation.

“Prime Minister Abe said to me, very specifically, ‘You are the guest of honour.’ There’s only one guest of honour … I’m the guest of honour at the biggest event that they’ve had in over 200 years,” he claimed. “So it’s a great thing.”