Former US national security advisor John Bolton said Sunday he thinks North Korean leader Kim Jong Un "gets a huge laugh" over US counterpart Donald Trump's perception of their relationship.

Bolton spoke to ABC News for his first interview ahead of the Tuesday release of his tell-all book, which contains many damning allegations against Trump.

When journalist Martha Raddatz asked if Trump "really believes Kim Jong Un loves him," Bolton replied he could see no other explanation.

"I think Kim Jong Un gets a huge laugh out of this," Bolton said. "These letters that the president has shown to the press... are written by some functionary in the North Korean Workers Party agitprop office.

"And yet, the president has looked at them as evidence of this deep friendship," he said, adding that friendship does not amount to international diplomacy.

Bolton also said he does not consider Trump to be fit for office and hopes he is a one-term president.

"I hope (history) will remember him as a one-term president who didn't plunge the country irretrievably into a downward spiral we can't recall from. We can get over one term," he said.

Bolton added that he will vote for neither Trump nor Democrat Joe Biden in the November presidential elections. Instead, he will "figure out a conservative Republican to write in" on the ballot.

Trump's administration had sought to halt publication of Bolton's book, but a US judge refused Saturday to block its release, saying it was too late for a restraining order.

Bolton's book also sharply criticizes South Korean President Moon Jae-in over his role in the Kim Jong Un summits.

Moon's office on Monday accused Bolton of distorting facts, jeopardizing future talks, and of violating "basic diplomatic principles."

- 'Last straw' -

"The Room Where it Happened" is Bolton's portrait of 17 months up close with Trump until he was fired last September.

In his interview, Bolton said he had resigned, noting that the "last straw" for him was when Trump invited the Taliban to Camp David during Afghan peace negotiations.

Bolton's book, which Trump describes as "fiction," describes the president "pleading" with Chinese President Xi Jinping during trade negotiations to boost the US president's chances of re-election.

Moreover, Bolton backs up the allegations at the center of Trump's impeachment last year that he pressured Ukraine to dig up dirt to weaken Biden's presidential bid.

Both Republican and Democratic lawmakers have criticized Bolton for publishing his book, saying he should have instead come forward during the impeachment process.

The House Intelligence Committee Chairman, Democrat Adam Schiff, told NBC's "Meet the Press" earlier Sunday that Bolton "indicts himself, for cowardice and for greed" by making his accusations in a book instead of testifying in front of the impeachment hearings.

Republican senator Tim Scott told ABC's "This Week" on Sunday that he also wished Bolton "would have come into the House under oath and testified."