WASHINGTON -- North Korean leader Kim Jong-un began putting conditions on denuclearization three months after his first-ever summit with US President Donald Trump in 2018, a soon-to-be-published book showed.

In his upcoming book, "Rage," Washington Post journalist Bob Woodward also noted that correspondence between Kim and Trump "increased in both frequency and affection" following their first summit held in June 2018.

"I just have arrived back in America, and the media for North Korea and you has been fantastic," Trump wrote to Kim on June 15, three days after the two met in Singapore in one of 27 letters exchanged between the two leaders disclosed in the Wooodward book.

In response, Kim wrote back on July 6, saying, "The significant first meeting with Your Excellency and the joint statement that we signed together in Singapore 24 days ago was indeed the start of a meaningful journey," according to excerpts of the book obtained by Yonhap News Agency.

Kim began putting conditions on denuclearization about three months later.

"We are willing to take further meaningful steps one at a time in a phased manner, such as the complete shutdown of the Nuclear Weapons Institute or the Satellite Launch District and the irreversible closure of the nuclear materials production facility," Kim said in his Sept. 6 letter.

It was not clear whether Kim had also listed his demands in return, based on what appears to be parts, not the entirety of his letter included in the book set to be published Tuesday.

Still, Woodward pointed out, "Historically the US had rejected conditions outright."

In their first meeting in Singapore, that also marked the first-ever US-North Korea summit, Trump and Kim signed a four-point agreement, under which North Korea reaffirmed its commitment to work toward complete denuclearization of the Korean Peninsula in exchange for a US security guarantee.

Trump and Kim met again in February 2019 in Hanoi for two days, but their talks ended without a deal.

Woodward offered an account of what went wrong, citing quotes from his 18 exclusive interviews with Trump held between December and July.

Kim was ready to give up one of his nuclear sites, but he had five, he writes.

"Listen, one doesn't help and two doesn't help and three doesn't help and four doesn't help. Five does help," Trump was quoted as telling him.

The two leaders met again on June 30, 2019, though only briefly at the Joint Security Area inside the Demilitarized Zone that separates South and North Korea.

They have yet to hold another meeting since despite exchanging more letters from then on, in one of which Kim expressed anger of the United States' joint military exercise with South Korea.

"My belief was that the provocative combined military exercises would either be cancelled or postponed ahead of our two countries' working-level negotiations where we would continue to discuss important matters," Kim said in a letter dated Aug. 5, 2019, according to Woodward.

"I am clearly offended and I do not want to hide this feeling from you. I am really, very offended," he added.

In a press conference immediately following their first summit in Singapore, Trump made a surprise announcement that the US would be ending its joint military exercises with South Korea.

"We will be stopping the war games, which will save us a tremendous amount of money, unless and until we see the future negotiation is not going along like it should," Trump had told the conference.

In November 2019, US Defense Secretary Mark Esper, together with his South Korean counterpart, also announced a decision to put off the allies' annual winter exercise "as an act of goodwill to contribute to an environment conducive to diplomacy and the advance of peace."

North Korea has yet to return to the dialogue table with the US. (Yonhap)