A "belligerent" letter from North Korean leader Kim Jong-un's right-hand man Kim Yong-chol prompted U.S. President Donald Trump to cancel Secretary of State Mike Pompeo's visit to Pyongyang, the Washington Post reported on Monday.
"Pompeo received the letter from Kim Yong--chol... on Friday morning, and showed it to Trump in the White House, two senior administration officials confirmed," according to the daily.
Trump tweeted, "We are not making sufficient progress with respect to the denuclearization of the Korean Peninsula."
"The exact contents of the message are unclear, but it was sufficiently belligerent that Trump and Pompeo decided to call off Pompeo's journey," the daily said.
CNN said the North Korean letter warned that denuclearization talks are "again at stake and may fall apart."
It came in response to a letter from Trump encouraging the North "to make more progress on denuclearization while warning against further backsliding," according to the Post.
"Trump isn't expected to totally abandon the diplomacy he began in his June 12 Singapore summit with Kim Jong-un. But unless Pyongyang responds positively to his latest move, Trump could side with officials who want to apply more pressure to the Kim regime through increased sanctions or other means, including National Security Adviser John Bolton," it said.
"Bolton and Defense Secretary Jim Mattis are opposed to Trump issuing a declaration of the end of the Korean War at this time."