Kim Jong-un also "expressed an intention to meet with Moon frequently in 2019" to pursue peace and "solve the issue of denuclearising the peninsula together", the spokesman said.
Moon Jae-in, in a message posted on Facebook, stressed Kim's willingness to comply with the South Korean and US agreements on the issue, but acknowledged that many obstacles remain to be overcome.
The report did not include details about the message but said it related to US-North Korea talks, and was conciliatory in tone. Unlike many other news organizations, we have not put up a paywall. There's no change in our heart about welcoming Chairman Kim (to the South)'.
Kim also met in 2018 with President TrumpDonald John TrumpFive challenges awaiting Mattis" successor Meet Trump's pick to take over the Joint Chiefs of Staff Retiring GOP lawmaker blasts Trump's threat to close border as "angry eighth-grader's tweet' MORE, and Trump has said he expects another meeting in early 2019.
Kim used his New Year's speech a year ago to initiate diplomacy with Seoul and Washington, which led to his meetings with Moon and a historic June summit with Trump.
Moon's tweet included photos of the top part of the letter, which began with: "Dear your excellency President Moon Jae-in".
"Chairman Kim Jong Un emphasized the bold step the two Korean leaders have taken to overcome the long-pending confrontation between North and South to meet three times in one year alone", read the letter, according to Kim Eui-kyeom, spokesman for the South's presidential office.
The U.S. stance toward North Korea appears to have softened recently before and after an inter-Korean ceremony to mark the launch of railway and road projects involving the North and South.
North Korea's leader Kim Jong Un and U.S. President Donald Trump could meet again in 2019.
The stalled negotiations had an impact on inter-Korean ties, including Kim's unrealized plan to visit Seoul this year as agreed their summit in Pyongyang in September.
Talks between the USA and North Korea have made little progress since Trump and Kim signed an agreement in June to "work toward complete denuclearization of the Korean Peninsula", without defining the term or setting a deadline. There continue to be doubts about whether Kim will ever voluntarily relinquish his nukes, which he may see as his strongest guarantee of survival.
Trump said last week that he was "looking forward" to his second summit with Kim, which the U.S. says may take place early next year.