Trump alluded to the letter in a late-night tweet thanking Kim for this week's return of remains of dozens of United States soldiers killed during the Korean War.
Vice President Pence said on Wednesday during a ceremony in Hawaii that the return of the remains is a sign of "tangible progress in our efforts to achieve peace on the Korean Peninsula".
Fifty-five cases of remains were airlifted from North Korea to South Korea last week and honored in a solemn ceremony at Joint Base Pearl Harbor-Hickam in Hawaii Wednesday.
The U.S. military sent 200 boxes to the demilitarized zone between North and South Korea in anticipation of the latest transfer, but North Korea handed over only 55 boxes, McKeague said, explaining that U.S. officials didn't know the specifics in advance.
The Post also reported that North Korean officials have talked about how they plan to deceive the USA about the size of their arsenal of missiles and nuclear warheads and facilities.
It's still unclear where the United States stands with North Korea.
Military equipment was also found among the remains, including boots, canteens, buttons and buckles.
"Some have called the Korean War the forgotten war, but today we prove these heroes were never forgotten", Pence said during his speech.
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The pledge to return the remains of US soldiers was made during a landmark summit between Trump and Kim in June in Singapore, where North Korea committed to work toward the complete denuclearization of the Korean peninsula.
Most of the remains returned to the USA were from the village of Sinheung Ri, near the Chosin Reservoir.
One dog tag from a U.S. Army soldier was among the recovered equipment, and officials said the soldier's family has been notified. "We couldn't be sure how many individuals were in each box".
More than 7,700 US troops remain unaccounted for from the Korea War.
"They had been very carefully packaged with padding and packaging that was done to, I think, a very high standard", he said.
The lab will begin by taking DNA samples from the remains and checking to see whether they match those of other remains in the inventory or those that family members of missing service members have provided. He said that DPAA has chest radiographs for about three-quarters of the missing from the Korean War.
The remains are now "undergoing forensic analysis" at the lab in Hawaii, another official said, adding that the U.S.is "guardedly optimistic" about the situation.