Top North Korean officials warned the United States in a letter that denuclearization talks are "again at stake and may fall apart," sources familiar with the process told CNN.
The letter was delivered to US Secretary of State Mike Pompeo, who abruptly canceled his fourth trip to Pyongyang, hours before he was scheduled to depart with his new special envoy Stephen Biegun on Friday, sources said.
Three sources with direct knowledge of the North Korean position on denuclearization said the letter stated that Kim's regime felt that the process couldn't move forward because "the US is still not ready to meet (North Korean) expectations in terms of taking a step forward to sign a peace treaty."
The US has so far been unwilling to replace the armistice agreement that ended the Korean War with a permanent, legally binding peace treaty, which would require the approval of two-thirds of the US Senate.
If a compromise cannot be reached and nascent talks crumble, Pyongyang could resume "nuclear and missile activities," the sources said.
The existence of the letter, sent to Pompeo by Kim Yong Chol, the former head of North Korea's spy agency, was first reported by the Washington Post.
The Post, which characterized the letter as a "secret" communication, said it wasn't clear how Kim had gotten the letter to Pompeo, but suggested that it may have been sent through its UN mission.
However, Pompeo has been relying on other backchannels, as opposed to dealing directly with his diplomatic counterpart North Korean Foreign Minister Ri Yong Ho, one source said.
"Before and after Pompeo became the Secretary of State, he seemed to be more interested in maintaining and engaging with Kim Yong Chol through a backdoor channel, rather than with his formal counterpart Ri Yong Ho," said the source.
The Post reported that the letter, shown to President Donald Trump by Pompeo, resulted in the US leader's decision to cancel Pompeo's scheduled trip to Pyongyang, Friday.
Pompeo and Andrew Kim, the head of the CIA's Korea Mission Center, were seen entering the West Wing of the White House on Friday afternoon just hours before Trump posted a series of tweets outlining his reasons for breaking off the visit.
"I have asked Secretary of State Mike Pompeo not to go to North Korea, at this time, because I feel we are not making sufficient progress with respect to the denuclearization of the Korean Peninsula," Trump tweeted on Friday.
"Secretary Pompeo looks forward to going to North Korea in the near future, most likely after our Trading relationship with China is resolved."
Pompeo was in the room when Trump sent his tweet canceling the trip, White House press secretary Sarah Sanders told CNN.
A senior diplomatic source told CNN that State Department officials were "briefing allies' embassies about their objectives for the trip like 10 minutes before" the trip was called off, and Pompeo himself had announced the Pyongyang visit just a day earlier.