JULY 1: Citizens were urged to be vigilant around objects that may have blown across the border from the South.
For years activists in the South have flown balloons across the border to send leaflets and humanitarian aid.
In response, Seoul says there is "no possibility" Covid could have crossed the border in that way.
According to North Korea's state media, an official investigation found two people who became infected with Covid early on in the outbreak after coming into contact with unidentified materials near the South Korean border.
An 18-year-old soldier and five-year-old child tested positive for the virus in early April after finding the objects on a hill in Ipho-ri, it reported.
Since then, state media said: "The malignant Covid-19 virus...has rapidly spread in the DPRK [North Korea]."
As a result of the investigation, people in the country are instructed to "vigilantly deal with alien things coming by wind and other climate phenomena and balloons in the areas along the demarcation line and borders."
Anyone who notices a strange object is instructed to report it immediately so it can be swiftly removed by an emergency anti-epidemic team.
Although the report did not mention South Korea directly, Seoul's unification ministry strongly denied the North's explanation for how Covid could have got into the country.
The closed border between the Koreas is one of the most heavily guarded in the world, but defectors and South Korean activists have for years launched balloons across the divide with anti-North Korean messages.
Since late April North Korea has been battling an explosive wave of 4.7 million cases of "fever," believed to be untested Covid infections.
Kim Jong-un described the outbreak in May as the greatest "turmoil to fall on our country since the founding", state media reported.
Until this year the reclusive nation had claimed to be completely Covid-free - although some experts believe the virus may have been circulating before then.
Its population of 25 million is vulnerable due to the lack of a vaccination programme and poor healthcare system, although in recent weeks there have been media reports that Pyongyang has accepted an offer of Chinese-manufactured vaccines.
It's not clear how many North Koreans, if any, have so far been vaccinated.
In the last few weeks officials say the number of new cases has dropped dramatically but many suspect the government of under-reporting the true figures.
The outbreak began some months after the North eased a strict lockdown of its border with China - with freight trains resuming their journeys between the two countries for the first time since early 2020.
Any suggestion that the virus could have entered the country this way, especially as China was dealing with a severe outbreak of the Omicron variant, would have raised difficult and diplomatically sensitive questions, North Korea expert Professor Lim Eul-chul told Reuters news agency.
"If they concluded the virus was from China they would have had to tighten quarantine measures on the border area in a further setback to North Korea-China trade," he said.
With inputs from BBC
MAY 13: Six people died after suffering a fever with one testing positive for Omicron, state media reported on Friday.
It said 187,000 people with a fever were being "isolated and treated".
While experts believe the virus has been present in the country for some time, the authorities only announced the first cases on Thursday.
They said there had been an outbreak of the Omicron variant in the capital, Pyongyang, and announced lockdown measures. They did not give precise case numbers.
But in an update on Friday, the official KCNA news agency reported that the outbreak extended beyond the capital. "A fever whose cause couldn't be identified spread explosively nationwide from late April," it said.
Around 350,000 people had shown signs of that fever, it added, without specifying how many had tested positive for Covid.
Analysts suggest the latest figures from state media, including the acknowledgement that the unspecified fever had spread nationwide, may indicate the country is experiencing an outbreak unlike any it has seen so far.
Its population of 25 million is vulnerable due to the lack of a vaccination programme and poor healthcare, experts say.
North Korea rejected offers from the international community to supply millions of AstraZeneca and Chinese-made jabs last year. Instead, it claimed it had controlled Covid by sealing its borders early in January 2020.
The country shares land borders with South Korea and China, which have both battled outbreaks. China is now struggling to contain an Omicron wave with lockdowns in its biggest cities.
On Friday, KCNA reported that the North Korean leader Kim Jong-un had visited a healthcare centre and "learned about the nationwide spread of Covid-19".
It described the situation as an "immediate public health crisis".
At a meeting outlining new Covid rules on Thursday, Mr Kim was seen wearing a face mask on television for what was believed to be the first time.
He ordered "maximum emergency" virus controls, which appeared to include orders for local lockdowns and gathering restrictions in workplaces.
There are fears a major outbreak could make it even more difficult for essential supplies to enter the country, leading to worsening food shortages and a faltering economy.
South Korea has said it offered humanitarian aid after Thursday's announcement, but Pyongyang is yet to respond.
Despite North Korea's earlier claims that it had "shining success" in keeping out Covid, there have been signs throughout the pandemic of its possible presence in the country including unconfirmed reports of cases and workers wearing hazmat suits.
