Caskets that the USA shipped to the border last month haven't been filled, despite Kim Jong Un's pledge during his June 12 summit with President Donald Trump to immediately repatriate identified remains.
Trump tweeted images of the letter, dated July 6, both in Korean and translated into English.
The White House had earlier said Mr Pompeo would meet with Mr Kim, but he did not turn up and he only met with Kim Yong Chol, the North Korean leader's powerful right-hand man.
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Roh Kyu-deok, a spokesman for South Korea's Foreign Ministry, said the South Korean government is "constantly in contact with the USA counterpart" about progress of the talks on war remains repatriation, but declined to comment on any details, saying that all inquiries on the issue be directed to the US authorities.
The U.S. says North Korea has continued to smuggle refined petroleum products into the country in excess of the quota of 500,000 barrels per year allowed under United Nations sanctions imposed because of nuclear and missile programs.
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. 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.
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Pompeo had said a Pentagon team would meet with the North's officials on or around Thursday at the inter-Korea border to discuss the repatriation.
While it was not immediately clear why the North Korean delegation skipped the meeting, the apparent snub reinforced skepticism around Pyongyang's willingness to commit to denuclearization of the Korean peninsula. "Great progress being made".
"We got back our great fallen heroes, the remains sent back today, already 200 got sent back", Trump said at a Minnesota rally last month.
Pompeo said after his trip that there had been progress in the talks.
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Mr Trump announced last month that North Korea has returned the remains of 200 United States troops, but there was no official confirmation of the move from military authorities.