(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.

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