Six people have died and 350,000 have been treated for a fever that has spread “explosively” across North Korea, state media said Friday, a day after acknowledging a COVID-19 outbreak for the first time in the pandemic.

The North’s Korean Central News Agency (KCNA) said of the 350,000 people who developed fevers since late April, 162,200 have recovered. It said 18,000 people were newly found with fever symptoms on Thursday alone, and 187,800 are being isolated for treatment.

One of the six people who died was infected with the Omicron variant, KCNA said. But it wasn’t immediately clear how many of the total illnesses were COVID-19.

North Korea imposed a nationwide lockdown Thursday after acknowledging its first COVID-19 cases. Those reports said tests from an unspecified number of people came back positive for the omicron variant.

North Korea likely doesn’t have sufficient COVID-19 tests and said it didn’t know the cause of the mass fevers. But a big coronavirus outbreak could be devastating in a country with a broken health care system and an unvaccinated, malnourished population.

KCNA said North Korean leader Kim Jong Un criticized officials for failing to prevent “a vulnerable point in the epidemic prevention system.” He said the outbreak was centered around the capital, Pyongyang, and stressed all work and residential units should be isolated from one another while residents should be provided every convenience during the lockdown.