The comments came after Mr Kim declared victory over Covid and ordered preventative measures to be eased just three months after acknowledging an outbreak.
The state Korean Central News Agency reported on Thursday that his sister said he had suffered a fever and blamed the country’s cases on leaflets flown across the border from South Korea.
Many experts believe the totalitarian regime has misrepresented the true scale of the virus in a bid to help maintain the Kim family’s control of the country.
South Korea’s Unification Ministry, which handles inter-Korean affairs, said it strongly regrets North Korea’s “extremely disrespectful and threatening comments" that were based on “ridiculous claims" about the source of its infections.
North Korea first admitted an outbreak in May and has since reported around 4.8million “fever cases” in a population of 26 million. It has named only a fraction of those as Covid cases, and insists only 74 people have died.
“Since we began operating the maximum emergency anti-epidemic campaign (in May), daily fever cases that reached hundreds of thousands during the early days of the outbreak were reduced to below 90,000 a month later and continuously decreased, and not a single case of fever suspected to be linked to the evil virus has been reported since July 29,” Mr Kim said in his speech on Wednesday, according to state media.
“For a country that has yet to administer a single vaccine shot, our success in overcoming the spread of the illness in such a short period of time and recovering safety in public health and making our nation a clean virus-free zone again is an amazing miracle that would be recorded in the world’s history of public health," he said.
His sister, Kim Yo Jong, called the country’s virus crisis a “hysteric farce" kicked off by South Korea to escalate confrontation.
She claimed that her brother had suffered fever symptoms and praised his “energetic and meticulous guidance" for bringing an "epoch-making miracle" in the fight against Covid-19.
“(South Korean) puppets are still thrusting leaflets and dirty objects into our territory. We must counter it toughly," she said.
“We have already considered various counteraction plans, but our countermeasure must be a deadly retaliatory one."