"I deeply appreciate the energetic and extraordinary efforts made by Your Excellency Mr. President for the improvement of relations between the two countries and the faithful implementation of the joint statement," the letter read, referring to an agreement the two leaders signed at their summit in Singapore last month."

"I firmly believe that the strong will, sincere efforts and unique approach of myself and Your Excellency Mr. President aimed at opening up a new future between the DPRK and the US will surely come to fruition," it added, using the acronym for North Korea's official name, the Democratic People's Republic of Korea.

"Wishing that the invariable trust and confidence in Your Excellency Mr. President will be further strengthened in the future process of taking practical actions, I extend my conviction that the epochal progress in promoting the DPRK-US relations will bring our next meeting forward."

In the joint statement, Kim committed to work towards the "complete denuclearization" of the Korean Peninsula in exchange for security guarantees from the US  Pompeo traveled to Pyongyang over the weekend to try to flesh out the deal, but apparently left with few tangible results. He later described his meetings with North Korean officials as "productive," while acknowledgnig that a lot of work remained to be done.

North Korea, meanwhile, lashed out at Washington for presenting "gangster-like demands" for complete, verifiable and irreversible denuclearization.

North Korea watchers downplayed the significance of the letter.

"It is in keeping with the North Korean approach to the talks with the US Instead of focusing on CVID, as the US would prefer, North Korea prefers to focus on building good relations and confidence building measures, which at some point in the future will lead to more stable relations and possibly a peace treaty," Ken Gause, a North Korea expert at CNA Corp., said in emailed remarks to Yonhap, referring to a peace treaty that would formally end the 1950-53 Korean War.

"Denuclearization is something the North Koreans are not focusing on other than being part of a general denuclearization of the Korean Peninsula, not unilateral denuclearization," he said, alluding to the North's longstanding demand that the US also remove its nuclear umbrella over South Korea.

Trump has repeatedly voiced optimism that Kim will dismantle the regime's nuclear weapons program despite recent news reports that North Korea continues to expand its nuclear facilities.

Speaking hours earlier at a press conference in Brussels, Belgium, Trump pointed to North Korea's suspension of nuclear weapons and ballistic missile testing in the past eight months as a sign of progress in efforts to denuclearize the regime.

"The decision to praise the letter seems more concerned with politics and pride than substantive progress with North Korea," said Adam Mount, a senior fellow and director of the Defense Posture Project at the Federation of American Scientists, where he covers North Korea.

"Actual progress requires supporting the negotiators working to hammer out an agreement, including our allies. It is bizarre to claim progress in a week when US teams have been ignored and insulted by North Korea," he said in an email.

Earlier Thursday the North Koreans postponed a meeting with US representatives at the inter-Korean border intended to discuss the repatriation of remains of American troops killed in the Korean War.

State Department spokeswoman Heather Nauert said the talks will take place Sunday and the US "will be prepared." The repatriation was part of Trump and Kim's agreement last month.

"Kim Jong-un is discovering that he can slow negotiations to a crawl so long as he placates the president, which is exactly the wrong message to send," Mount said. "Progress will be impossible so long as the president praises obstruction." (Yonhap)