Despite the apparent snub and worsening relations with North Korea, Donald Trump tweeted on Monday he has "confidence" that Mr Kim would honour the denuclearisation "contract" they signed at the Singapore summit.
The date of the letter suggests it may have been sent around the time of Secretary of State Michael Pompeo's visit to Pyongyang last week, in which the United States and North Korea struggled to make headway on the denuclearization issue.
North Korean officials have failed to turn up for talks with a U.S. team on the repatriation of the remains of American soldiers killed in the Korean war.
Kim also agreed to repatriate remains of USA troops who died during the Korean War six decades ago.
North Korea has imposed a testing moratorium, with the last missile test occurring on November 28, and it destroyed its main nuclear testing facility, although experts say those moves could be reversed.
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In another apparent setback, the U.S. accused Pyongyang of violating sanctions on refined oil products and ordered the United Nations security council to immediately halt all related deliveries to North Korea.
But readers soon pointed out that the letter didn't actually mention any action that had been taken, and only made vague promises about 'the future progress of taking practical actions'.
"We were ready", the official said.
There was no indication of whether the North Koreans intentionally blew off the meeting. The meeting was about returning the remains of American soldiers from the Korean War.
During an official visit to Britain, Trump posted images of the Korean-language letter from Kim and its English translation. That's according to documents seen by The Associated Press on Thursday and sent by the U.S.to the Security Council committee monitoring the sanctions.
But while improving relations and decreasing hostility has certainly been a benefit of Trump's diplomatic push on North Korea, it's been a means to the end of denuclearization.
Pompeo characterized the talks as progressive, asserting that North Korea did not "push back" on denuclearization. In fact, it appears to echo concerns that Trump did nothing more than give up a bunch of concessions without having Kim commit to a hardline stance on dismantling his country's nuclear ambitions.