The North Korean leader sent South Korean president Moon Jae-in a “message of comfort to the South Korean people who are battling against the outbreak of Covid-19,” according to Moon’s senior secretary for public communication, Yoon Do-han.
Kim expressed confidence that South Korea, which has confirmed over 5,000 cases of the illness, would “prevail in this fight without fail” and offered his “unwavering friendship and trust”.
The message is in stark contrast to Kim’s younger sister’s recent comments to South Korea, after they protested against the North’s latest live-fire exercises.
Kim Yo-jong is in charge of propaganda affairs for North Korea and has frequently appeared at her brother's major public events including summits with Donald Trump and other regional leaders.
But her statement carried by state media was the first of its kind and indicated a further elevation of her political status.
In the statement issued on Tuesday night, she criticised South Korea's presidential Blue House for expressing strong concerns over the North's firing drills and urging it to stop acts that do not help reduce military animosities.
"As far as I know, the South side is also fond of joint military exercises and it is preoccupied with all the disgusting acts like purchasing ultra-modern military hardware," Ms Kim said.
"They meant they need to get militarily prepared but we should be discouraged from military exercises.
"Such a gangster-like assertion can never be expected from those with normal way of thinking.”
Describing the Blue House as "a mere child" and "a burnt child dreading fire", she questioned how its words and actions could be "so perfectly foolish in detail”.
Ms Kim did not name liberal South Korean President Moon Jae-in, whom she has met several times.
She only said: "The South side's response is so regretful and disappointing but it is somewhat fortunate that it was not direct statement of the president.”
South Korea's Unification Ministry, which handles inter-Korean affairs, said it has no specific comment on Ms Kim's statement.
But spokesman Yoh Sang-key said the two Koreas should maintain mutual respect while working toward establishing a peace on the Korean Peninsula.