Kim gave the directive after his historic summit with U.S. President Donald Trump in Singapore last month, said Choi Sung-yong, head of a group representing families of South Koreans abducted by North Korea, citing an informed source in the North's capital.
Trump shared the letter in a tweet Thursday, writing, "A very nice note from Chairman Kim of North Korea. Great progress being made!"
But what can't be denied is that Trump's decision to end the annual US/South Korea "war games" (long used by the DPRK to justify its nuclear defence program), and his statement that "yesterday's conflict does not have to be tomorrow's war", elevated Kim's isolated regime from enemy to potential friend.
In it, Mr Kim says: "I deeply appreciate the energetic and extraordinary efforts made by Your Excellency Mr President". Pyongyang has asked the UN Command Armistice Commission to upgrade the talks to a higher level to include a US general, South Korea's foreign ministry said.
Mr Pompeo travelled to Pyongyang for two days last week with the aim of fleshing out denuclearisation commitments, insisting the talks were making progress.
North Korea, meanwhile, lashed out at Washington for presenting "gangster-like demands" for complete, verifiable and irreversible denuclearization.
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.
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Last month, Trump told a Republican convention in Las Vegas that North Korea had "already sent back, or are in the process of sending back" the remains of 200 United States or allied service members following his summit with Kim.
President Trump tweeted out the letter and an English translation on Thursday. He later described his meetings with North Korean officials as "productive", while acknowledgnig that a lot of work remained to be done.
The Defense Department estimates there are up to 5,300 sets of US service member remains still somewhere in North Korea.
Representatives of the two countries had been poised to meet in Panmunjom, which is regarded as the truce village on the inter-Korean border. Roughly 5,300 USA troops presumed to have been killed in the Korean war are unaccounted for.
The United States requested the UN sanctions committee to declare that North Korea had violated the UN-approved quota and "order an immediate halt to all transfers of refined petroleum products" to North Korea.
A USA official waited at the border truce village of Panmunjom but no North Korean official showed up, South Korea's Yonhap news agency said, citing a Seoul diplomatic source.
But skepticism remains over whether North Korea's findings will satisfy Japan.