North Korea confirmed its first ever Covid-19 death on Friday, saying fever was spreading "explosively" nationwide and tens of thousands of people were being isolated and treated after falling sick.

The nuclear-armed country only reported its first Covid cases Thursday, saying it was moving into "maximum emergency epidemic prevention system" after sick patients in the capital Pyongyang tested positive for Omicron.

North Korea has been under a rigid coronavirus blockade since the start of the pandemic in 2020, but with massive Omicron outbreaks in all neighbouring countries, experts said it was only a matter of time before Covid snuck in.

"A fever whose cause couldn't be identified explosively spread nationwide from late April," the official Korean Central News Agency said.

"Six persons died (one of them tested positive for the BA.2 sub-variant of Omicron,)" it added.

With its 25 million people not vaccinated against Covid, North Korea's crumbling health infrastructure would struggle to deal with a major outbreak, experts say.

"On May 12 alone, some 18,000 persons with fever occurred nationwide and as of now up to 187,800 people are being isolated and treated," KCNA said.

Leader Kim Jong Un -- seen wearing a mask on state TV for the first time -- oversaw an emergency meeting of the Politburo on Thursday and ordered nationwide lockdowns in a bid to halt the spread of the virus.

On Friday, KCNA said Kim visited the state emergency epidemic prevention headquarters and "learned about the nationwide spread of Covid-19".

"It is the most important challenge and supreme tasks facing our Party to reverse the immediate public health crisis situation at an early date," KCNA added.

It is likely that the massive nationwide outbreak is linked to a huge military parade held in Pyongyang on April 25, said Cheong Seong-chang of the Sejong Institute.

"Holding a military parade attended by a large crowd, when Omicron was raging in neighboring China, shows Pyongyang was overconfident in their capabilities to fight and prevent the virus," he told AFP.

North Korea was likely to see "major chaos" due to the rapid spread of Omicron, he said, given that the country is currently reporting nearly 20,000 cases in a single day.

"If the death toll from Omicron spikes, Pyongyang may have to ask for China's support," he added.

Beijing, Pyongyang's sole major ally and benefactor, said Thursday that it was ready to assist North Korea with its Covid-19 outbreak. 

But China, the world's only major economy to still maintain a zero-Covid policy, is itself battling multiple Omicron outbreaks -- with some major cities, including financial hub Shanghai, under strict stay-at-home orders.

North Korea has previously turned down offers of Covid vaccines from China, as well as from the World Health Organization's Covax scheme.

Kim said Friday that the outbreak of fever "shows that there is a vulnerable point in the epidemic prevention system" and called for more lockdowns.

Kim "said that it is the top priority to block the virus spread by actively locking down areas and isolating and treating persons with fever in a responsible manner," KCNA reported.

Analysts said that China's experience with Omicron indicated lockdowns might not be successful, but with no antiviral treatment or vaccines, North Korea has few other options.

North Korea test-fired three short range ballistic missiles, Seoul said Thursday -- shortly after confirming their first cases of Covid.

New South Korean President Yoon Suk-yeol's administration slammed the North's "continuing provocations with a ballistic missile launch despite the outbreak of coronavirus," his security office said after a meeting.

After high-profile talks collapsed in 2019, North Korea has doubled down on weapons testing, conducting a blitz of launches so far this year, including intercontinental ballistic missiles.

Satellite imagery indicates North Korea is preparing to conduct a nuclear test, and the United States has warned this could come as soon as this month.

If Pyongyang needs aid -- vaccines and medicine -- they might need to delay the test, some analysts said, but others warned the Covid-19 outbreak could hasten things along.

"A nuclear test would be a good way to distract the public from the pandemic," Yang Moo-jin, a professor at the University of North Korean Studies, told AFP.