North Korea's senior official Kim Yong-chol reportedly told U.S. Secretary of State Mike Pompeo not to visit Pyongyang unless he is willing to offer something.
Reuters quoted a senior U.S. official on Thursday as saying that the letter to Pompeo said that North Korea believed the U.S. was not doing enough.
Reuters said that the letter convinced U.S. officials to distrust the North's commitment to its denuclearization and to lose hope for positive results from Pompeo's planned trip.
The report said that U.S. officials are concerned that North Korea may attempt to strike a separate deal with South Korea, in a bid to drive a wedge between the South and the U.S.
According to CNN on Tuesday, North Korea warned in the letter that denuclearization talks were "again at stake and may fall apart."