"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", reads the letter dated July 6.
According to estimates from the Pentagon, 7,700 USA troops are still unaccounted for from the Korean War.
"He's realized he can humiliate and ignore the U.S.as long as he flatters the president", Mount said.
The US military has since announced that 100 wooden coffins have been moved to the inter-Korean border in preparation.
Several days previously, Pyongyang had slated the USA for seeking unilateral and forced denuclearization from North Korea.
With the meeting not taking place, the UNC reportedly made a phone call and North Koreans responded by saying that they needed to upgrade the "level" of the talks.
That number includes 7,702 who are missing in action, with an estimated 5,300 believed to have been lost in the North, according to the Hawaii-based DPAA, which oversees the effort.
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The North Korean leader said he hopes to "bring our next meeting forward" in the concluding sentence of the letter.
Earlier Thursday, a diplomatic source here said the USA and North Korea were still in talks to set a date for working-level dialogue over the repatriation of remains of American troops.
But in stark contrast, Pyongyang's take was overwhelmingly negative, with the North warning that the future of the peace process was being jeopardized by "unilateral and gangster-like" US demands for its nuclear disarmament.
"We have to see the USA defense ministry's response, but the UNC reportedly gave a positive signal to North Korea, so there is a good chance of the talks taking place on Sunday", the source said.
Mr. Pompeo denied that the talks went poorly, telling pool reporters that progress was made "in every element of our discussions".
North Korean and United States teams, as well as UN officials, had been due to discuss the details of repatriation.
Mr Kim agreed in broad terms to "work toward denuclearisation" of the Korean Peninsula when he met Mr Trump in Singapore last month.
Mount said Trump's reaction would be a concern.