US President Donald Trump went into his 2018 summit with North Korean leader Kim Jong Un in Singapore viewing it as a publicity exercise, and was prepared to sign a substance-free joint statement, said former national security adviser John Bolton in his upcoming memoir.

The 577-page book, titled The Room Where It Happened, is an account of Mr Bolton's 17 months in the Trump administration and details the President's dealings with several foreign leaders, including his three meetings with Mr Kim.

The former top official is highly critical of Mr Trump in his book, calling him uninformed and accusing him of putting his personal interests over the nation's.

The memoir was scheduled to be released yesterday, in defiance of legal challenges by the Trump administration, but excerpts have already been published by the media.

The Trump-Kim summit in Singapore on June 12, 2018, was the first time a sitting US president and North Korean leader had met.

It was hailed as a historic step forward for relations between the two countries and raised hopes for North Korean denuclearisation, though critics at the time noted that the joint statement was too vague and lacked a concrete timeline and process.

Mr Bolton, who was against the meeting from the beginning and repeatedly hoped it would be cancelled, sides with the critics and wrote that the President was more interested in the optics than the substance of the historic summit.

Mr Trump wanted to go to Singapore, saying "it will be great theatre", according to Mr Bolton. On the flight to Singapore, Mr Trump was also obsessed with watching press coverage of the North Korean leader's arrival in the Republic and with what the coverage of his own arrival would be.

He was also satisfied with the short statement that negotiators had come up with, even though the text "didn't say much of anything" and came nowhere near declaring an end to the Korean War, according to Mr Bolton.

When the two met at the Capella Singapore hotel in Sentosa, Mr Kim flattered Mr Trump, saying that his predecessors would not have shown the leadership to hold the summit. In response, said Mr Bolton, "Trump preened".

Mr Kim also asked Mr Trump how he assessed him. The President replied that he saw him as "really smart, quite secretive, a very good person, totally sincere, with a great personality", said Mr Bolton.

But the two leaders were not on the same page at the end of the summit, with Mr Kim seeming to think that he and Mr Trump had agreed on an "action for action" approach, in which North Korea would denuclearise in phases as the US simultaneously rolled back its sanctions.

Mr Bolton also portrayed South Korean President Moon Jae-in as having set up unrealistic expectations for both Pyongyang and Washington in pursuit of his agenda of Korean reunification. Mr Moon told Mr Trump that Mr Kim had committed to complete denuclearisation, said Mr Bolton.

On Monday, South Korea said that the book was inaccurate and substantially distorted facts.

South Korean national security adviser Chung Eui-yong said in a statement: "Unilaterally publishing consultations made based on mutual trust violates the basic principles of diplomacy and could severely damage future negotiations."

Mr Trump has slammed Mr Bolton and called his tell-all full of lies and fake stories.

In her own upcoming book, former White House press secretary Sarah Sanders accuses Mr Bolton of acting "like he was the president, pushing an agenda contrary to President Trump's", Politico reported on Monday.

Dr Sue Mi Terry, the Centre for Strategic and International Studies (CSIS) senior fellow for Korea, said the Singapore chapter has put North Korea's recent provocations in a new light.

"I can see why Kim Jong Un is so upset now," she said on a CSIS podcast discussing the book. "He was just under the wrong impression, he thought we were going to go differently after Singapore."

CSIS senior adviser Victor Cha, a former director for Asian affairs at the National Security Council, said that Mr Bolton's revelations in the book dovetailed with what people involved in policy had told him.

"Trump really didn't care about substance, it was all about the show," said Dr Cha.

"There was no interest in the underlying substance of what was being discussed. It was just the narrative that he's going to go and change things, that he's different. And he got the show."