"The North's leader said he will quietly support South Korea's fight against COVID-19 and showed his constant friendship and trust toward Moon," Yoon Do-han, the office’s spokesperson, said, as quoted by the Yonhap news agency.
Kim also expressed concern about Moon's health and said he believed South Korea would overcome the outbreak.
The chairman further shared his "frank" thoughts with regard to the situation on the Korean Peninsula, according to the South Korean presidential office.
Moon has already replied to the letter, in which he expressed gratitude to his North Korean counterpart, the spokesperson added.
The message, delivered on Wednesday, came amid reports that the number of COVID-19 cases in South Korea had risen by 438 to 5,766. The death toll has also gone up to 35. North Korea has not yet reported any coronavirus cases in the country.