North Korea's Kim Jong Un declared a "shining victory" over Covid-19 as his sister revealed he had fallen ill during the outbreak, which she blamed on Seoul, state media said on Thursday.

Addressing a meeting of health workers and scientists, Kim announced a "victory... in the war against the malignant pandemic disease", according to the official Korean Central News Agency.

The isolated country, which has maintained a rigid coronavirus blockade since the start of the pandemic, confirmed an Omicron outbreak in the capital Pyongyang in May and activated a "maximum emergency epidemic prevention system".

North Korea refers to "fever patients" rather than "Covid patients" in case reports, apparently due to a lack of testing capacity.

It has recorded nearly 4.8 million "fever" infections and just 74 deaths for an official fatality rate of 0.002 percent, according to state media. It has reported no new cases since July 29.

This handling of the pandemic "is a miracle unprecedented in the world's public health history," Kim said to thunderous applause, according to KCNA. "The victory gained by our people is a historic event."

Kim's powerful sister, Kim Yo Jong, said the top leader himself had been ill during the outbreak, according to another KCNA report.

Kim "was suffering from high fever during the days of this quarantine war, but he could not lie down for a moment as he was thinking about the people he was responsible for", his sister said.

This is the first time North Korea has indicated its leader had been infected by the coronavirus.

Yo Jong also claimed the country's Covid-19 outbreak was caused by South Korea, warning of "retaliation".

North Korea has previously said that "alien things" near the border with the South caused the isolated country's Covid outbreak, a claim Seoul has rejected.

Despite a ban that took effect in 2021, South Korean activists have for years flown balloons containing propaganda leaflets and US dollars over the border, which Pyongyang has long protested against.

Seoul's Unification Ministry on Thursday said North Korea was repeating a "groundless claim" and expressed regret that Pyongyang was making "rude and threatening remarks". (AFP)