Korean Central News Agency
Now playing
01:24
Sources: N. Korea warns progress may fall apart
Nicolas Asfouri/Pool/Getty Images/AFP PHOTO/KCNA VIA KNS
Now playing
01:27
North, South Korean leaders to meet again
Airbus Defense and Space
Now playing
01:44
New images show N. Korea dismantling test site
CNNI
Now playing
00:40
Pompeo dismisses N. Korea's 'gangster' comments
SINGAPORE - JUNE 12: In this handout photo, North Korean leader Kim Jong-un shakes hands with U.S. President Donald Trump during their historic U.S.-DPRK summit at the Capella Hotel on Sentosa island on June 12, 2018 in Singapore. U.S. President Trump and North Korean leader Kim Jong-un held the historic meeting between leaders of both countries on Tuesday morning in Singapore, carrying hopes to end decades of hostility and the threat of North Korea's nuclear program. (Photo by Kevin Lim/THE STRAITS TIMES/Handout/Getty Images)
Handout/Getty Images AsiaPac/Getty Images
SINGAPORE - JUNE 12: In this handout photo, North Korean leader Kim Jong-un shakes hands with U.S. President Donald Trump during their historic U.S.-DPRK summit at the Capella Hotel on Sentosa island on June 12, 2018 in Singapore. U.S. President Trump and North Korean leader Kim Jong-un held the historic meeting between leaders of both countries on Tuesday morning in Singapore, carrying hopes to end decades of hostility and the threat of North Korea's nuclear program. (Photo by Kevin Lim/THE STRAITS TIMES/Handout/Getty Images)
Now playing
01:56
Kim Jong Un snubbed Mike Pompeo, source says
WASHINGTON, DC - MAY 09:  National Security Adviser John Bolton speaks on a morning television show from the grounds of the White House, on May 9, 2018 in Washington, DC. Yesterday President Donald Trump announced that America was withdrawing from the Iran nuclear deal.  (Photo by Mark Wilson/Getty Images)
Mark Wilson/Getty Images
WASHINGTON, DC - MAY 09: National Security Adviser John Bolton speaks on a morning television show from the grounds of the White House, on May 9, 2018 in Washington, DC. Yesterday President Donald Trump announced that America was withdrawing from the Iran nuclear deal. (Photo by Mark Wilson/Getty Images)
Now playing
01:34
Bolton: US has plan for denuclearizing N. Korea
Planet Labs Inc.
Now playing
01:25
Satellite images show missile plant construction
CNN
Now playing
01:14
Susan Rice: Kim Jong Un beat Trump at summit
Images of the Norrth Korea missile launch on November 28 taken from Rodong Sinmun, North Korea's official newspaper.
From Rodong Sinmun
Images of the Norrth Korea missile launch on November 28 taken from Rodong Sinmun, North Korea's official newspaper.
Now playing
02:14
Will North Korea restart nuclear tests?
Photo Illustration/Getty Images
Now playing
03:00
Will Kim Jong Un ever give up his nukes?
Photo Illustration/Getty Images
Now playing
02:27
What's bringing Kim Jong Un to the table
Now playing
01:51
Who is Kim Jong Un?
CNN
Now playing
01:43
Connolly: Trump comment on Kim 'jaw-dropping'
Now playing
02:31
Moon: The masterful dealmaker
Trump Kim Jong Un comment 04240218
CNN
Trump Kim Jong Un comment 04240218
Now playing
01:26
Trump: Kim Jong Un very open and honorable
Now playing
03:06
Finding art on the edge of the DMZ
(CNN) —  

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, whose fourth trip to Pyongyang was abruptly canceled, 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.

North Korea accuses US of ‘hatching a criminal plot to unleash a war’

02:40 - Source: CNN
Kim goes casual while under US pressure

Abruptly canceled

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 on 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.

CNN’s Zachary Cohen and Jeremy Diamond contributed to this report.