Trump mentioned the letter in a tweet earlier Thursday as he thanked Kim for returning the remains of presumed American soldiers killed in the 1950-53 Korean War.
On Wednesday, 55 sets of remains were repatriated to the USA, where they were received at a ceremony in Hawaii.
United States officials have been closely monitoring North Korea's willingness to abandon its nuclear ambitions.
While it is hoped that the repatriation will give momentum to the implementation of the key element of the deal - the dismantlement of North Korea's nuclear weapons program - progress toward securing firm commitments in that regard from Pyongyang has reportedly been slow.
Byrd, who went to Wonsan in North Korea last week as part of the team that brought back the remains, said he was told by North Korean officials that the remains were recovered from the village of Sin Hung-ri on the east side of the reservoir.
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With the remains having been returned to the United States, the Defense P.O.W./M.I.A. Accounting Agency, a Department of Defense outfit, now has the hard task of identifying them, which could take months, according to The New York Times.
More than 7,700 USA troops remain unaccounted for from the Korea War.
"Everything we saw was consistent with these remains, indeed, being from the Korean War, and consistent with these remains being good candidates to be missing Americans from the Korean War", said Byrd.
Most of the remains returned to the USA were from the village of Sinheung Ri, near the Chosin Reservoir. USA officials say they have already contacted the family of a service member whose dog tag was returned.
Byrd cautioned that the 55 boxes did not necessarily equate to the remains of 55 individuals.
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Humanitarian conditions in Gaza have steadily declined in recent years and it's economic state is dire. At the time, Israel's foreign ministry said it had provided the flotilla with permission.
"They had been very carefully packaged with padding and packaging that was done to, I think, a very high standard", he said.
As he secured a commitment for the complete denuclearization of the Korean Peninsula, @POTUS also secured a promise from Chairman Kim to return the remains of all fallen USA service-members lost in North Korea.
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.
But other techniques such as comparing teeth to dental records and bones to chest X-rays are also used.
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