And North Korea has not shown an inclination to share the economic data that the International Monetary Fund would require for membership.
"We call on all nations to maintain decisive pressure on (North Korea), including by fully implementing existing U.N. sanctions", the statement said.
What was in Kim's letter?
In it, Mr Kim says: "I deeply appreciate the energetic and extraordinary efforts made by Your Excellency Mr President".
The North Korean leader also expressed optimism about a future meeting.
"I extend my conviction that the epochal progress in promoting the DPRK-U.S. relations will bring our next meeting forward".
The letter's date indicates it was written before Secretary of State Michael Pompeo's visit late last week to Pyongyang, where the USA and North Korea struggled to agree over plans to denuclearise the Korean peninsula. He did not elaborate.
On Sunday, he brushed off North Korean charges that he used "gangster-like" diplomacy in the negotiations and vowed that he would keep pursuing denuclearization talks with Pyongyang.
Secretary of State Mike Pompeo ignored a question about the apparent snub when asked by reporters traveling with him in Brussels on Thursday.
The president of the United States confirmed on Tuesday morning that he plans to give a copy of the Elton John hit song "Rocket Man" to the dictator of North Korea, who he met with during a summit in Singapore last month to discuss denuclearization. Kim mentioned that Trump had referred to him as "Rocket Man" a year ago when North Korea was engaging in nuclear tests and missile launches.
What about the planned repatriation meeting?
Representatives of the two countries had been poised to meet in Panmunjom, which is regarded as the truce village on the inter-Korean border.
"Both the USA and the North are still in discussion over when to meet", the official added.
Earlier Thursday, after attending the NATO summit in Brussels, he told reporters that there were clear signs of progress with North Korea, most notably that no missile and nuclear tests have occurred for nearly nine months.
Trump told a crowd of supporters a week later that the remains of 200 Americans "have been sent back".
The North's criticism was a "strategy" meant to show its frustration about what it sees as a lack of action from the United States in response to the steps it had recently taken, Moon said.
If realized, they will be the first general-level military talks between North Korea and the UNC since March 2009.
The Pentagon says Pyongyang has indicated several times they have as many as 200 sets of remains that could be those of United States soldiers who died in the war.