At the time of the talks on Sunday, U.S. officials were unfamiliar with Kim Myong-gil's name, but later conducted an inquiry to look over his background, various sources familiar with the meeting said.
While the talks, which began in 2003, reached several agreements related to the dismantlement of the North's nuclear program, Pyongyang eventually walked out after disagreements over implementation and verification of the agreed-upon denuclearization process.
What did North Korea say? "We've done a lot", he said.
It also accused Washington of attempting to "undermine the tranquil atmosphere" on the Korean peninsula.
The delegation claimed the letter - also signed by Britain, France and Germany - was sent on June 29, the same day Trump tweeted that he would like to shake Kim's hand during his visit to the Korean peninsula.
Last month, the US told a council sanction committee that North Korea had breached a cap on refined petroleum imports, mainly through transfers between ships at sea.
US President Donald Trump's meeting with North Korean leader Kim Jong Un in the demilitarised zone (DMZ) between the Koreas last Sunday was planned ahead by the two sides, the Asahi has reported, citing unnamed US and North Korean diplomats.
The U.S. letter to United Nations member states, which was reportedly sent on June 27, urged countries to call for the return of all North Korean workers by the end of 2019.
Trump told the official to discuss the matter with North Korea and received confirmation that Pyongyang would send a predetermined signal if Kim was willing to meet at the DMZ, the sources said.
"It is quite ridiculous for the United States to continue to behave obsessed with sanctions and pressure campaign against the DPRK, considering sanctions as a panacea for all problems", North Korea's mission said in a statement.
"The foreign ministry spokesman is saying that North Korea and the United States are the main parties and China is an important stakeholder".
The call came after a series of events in recent weeks that have sent positive signals about a possible restart of the negotiations between the U.S. and North Korea on the denuclearisation of the Korean peninsula, after a summit between Trump and Kim in Hanoi in February was cut short.
On the face of things, neither side has fundamentally shifted its position, though there are reports that inside the Trump team there are discussions about pursuing a more limited initial objective to freeze Pyongyang's nuclear programme rather than to seek full denuclearisation. With the UFD's role apparently reeled back, the Foreign Ministry, under Foreign Minister Ri Yong-ho, appeared to take a leading role in standing by Kim Jong-un during Sunday's abrupt summit.
Mr Trump - who as soon as referred to Mr Kim as "little rocket man" - referred to as their friendship "particularly sizable" and said it used to be a "sizable day for the enviornment".
However, negotiations stalled after a failed February summit at which Trump insisted that Kim give up his entire nuclear programme in exchange for sanctions relief.