WASHINGTON -- US President Donald Trump said in letters to North Korean leader Kim Jong-un last year that only they can resolve hostility between their countries and promised to be his friend forever, Washington Post journalist Bob Woodward writes in his upcoming book.

"Only you and I, working together, can resolve the issues between our two countries and end nearly 70 years of hostility, bringing an era of prosperity to the Korean peninsula that will exceed all our greatest expectations -- and you will be the one to lead," Trump said in the letter June 12, 2019, according to excerpts of the new book "Rage," obtained by Yonhap News Agency.

Trump's letter came about four months after he and Kim held their second bilateral summit in Hanoi in February, which ended without a deal. Trump said in the letter that he wanted to meet Kim again, according to the book.

The Watergate reporter is said to have obtained 27 letters exchanged between Trump and Kim, 25 of which have never been disclosed to the public. The book is set to hit bookshelves Tuesday.

Trump's June 12 letter was a reply to a letter from Kim, dated June 10, which Woodward describes as a "letter of verbose flattery."

"Like the brief time we had together a year ago in Singapore, every minute we shared 103 days ago in Hanoi was also a moment of glory and remains a precious memory," Kim wrote, according to the book.

"I also believe that the deep and special friendship between us will work as a magical force," the North Korean leader was quoted as saying.

Such flattery came from a man who, according to the book, had offered to Trump a very graphic account of how he had his own uncle executed.

Trump explained to Woodward that Kim had Jang Song-thaek, Kim's uncle by marriage, decapitated, with his body put on display on the steps of a building used by senior North Korean officials.

It was not immediately clear when Kim had offered such a vivid account of Jang's execution.

Still, Trump promised to always be Kim's friend, even after their second summit in Hanoi fell through.

"Thank you again for making this long journey to Hanoi. As I said to you when we parted ways, you are my friend and always will be," Trump wrote in another letter to Kim, dated March 22, 2019.

Kim and Trump met again in June 2019 at the joint village of Panmunjom inside the Demilitarized Zone that divides the South and North Korea.

Their talks have stalled since then. (Yonhap)