North Korean leader Kim Jong Un sent a personal letter to "comfort" South Koreans fighting the novel coronavirus epidemic just a day after his sister condemned Seoul as a "frightened dog barking", the South's presidential office said Thursday.

Pyongyang has imposed strict restrictions and closed its borders to try to prevent an outbreak and insists it has not had a single case of the novel coronavirus which emerged in the Chinese city of Wuhan and has since spread around the world.

The South has reported the largest number of cases in the world outside China and its total passed 6,000 Thursday.

In his letter to President Moon Jae-in, Kim "conveyed his message of comfort to the South Korean people who are battling against the outbreak", said Yoon Do-han, a senior official at the presidential Blue House.

Kim said he was "worried" about Moon's own health, Yoon added, and "underlined his unwavering friendship and trust towards" Seoul.

It was a marked contrast to the statement issued on Tuesday by Kim's sister Kim Yo Jong, one of his closest advisers, when she said Seoul's condemnation of military drills by the North was a "truly senseless act" and "perfectly foolish".

She likened it to "a frightened dog barking".

"Such incoherent assertion and actions made by Chongwadae only magnify our distrust, hatred and scorn for the south side as a whole," she added, referring to the Blue House.

On Monday the nuclear-armed North fired what the South's military said appeared to be two short-range ballistic missiles, which it is banned from testing under UN Security Council resolutions that impose multiple sanctions over its weapons programmes.

The launch came days after the one-year anniversary of the collapsed Hanoi summit between US President Donald Trump and Kim. Negotiations have since been at a standstill and Pyongyang has repeatedly lashed out at the South, saying it has "nothing to talk" about any more with Seoul.