North Korea struggles with chronic food shortages, which have been exacerbated by a years-long self-imposed coronavirus blockade, coupled with biting international sanctions over its weapons programmes.

Despite state media reports claiming Covid is under control, the World Health Organization warned last week they "assume that situation is getting worse not better".

Experts have said the outbreak could trigger a major health crisis in the country, which has one of the world's worst healthcare systems.

North Korea reported its first Omicron cases on May 12 and the virus has since torn through its unvaccinated population of 25 million, with state media confirming Thursday more than 4.3 million cases of "fever" in total.

"As required by the maximum emergency anti-epidemic system, we demand all staff strictly abide by anti-pandemic rules and regulations," sanitation official Kim Hye Kyong told AFP in Pyongyang Thursday as hazmat-clad workers sprayed down trolley buses.

Early in the pandemic, Pyongyang repeatedly rejected offers of Covid vaccines, including from the WHO, and more recently has ignored new offers of medical assistance and jabs from Seoul and Washington.

Pyongyang's state media -- which typically reports on successful weapons tests 24 hours later -- has not reported on any of the country's recent missile launches.

This means most North Koreans will know very little "about how many resources their government has been blasting into the sea", Sokeel Park, South Korea country director at Liberty in North Korea, said on Twitter.

North Koreans have been kept in the dark about military spending even as they deal "with the pandemic, shortages from two years of lockdown, and skyrocketing medicine prices", he added.

USA and South Korean officials also have been warning for weeks that Kim's regime is preparing to carry out a fresh nuclear test.

Wendy Sherman, US deputy secretary of state, on Tuesday said there would be a "swift and forceful" response if Pyongyang goes ahead with its seventh nuclear test.