North Korean authorities imposed a lockdown on the border city of Kaesong after discovering what they say is the country's first suspected coronavirus case, state media reported Sunday.
Leader Kim Jong-un convened an emergency politburo meeting on Saturday to introduce a "maximum emergency system and issue a top-class alert" to contain the virus, official news agency KCNA said.
If confirmed, it would be the first officially recognised case of Covid-19 in North Korea, where the healthcare infrastructure is thought to be woefully inadequate to deal with an epidemic.
KCNA said a defector who left for South Korea three years ago returned on July 19 after "illegally crossing" the heavily fortified border dividing the two countries.
But there have been no reports in the South of anyone leaving through what is one of the world's most secure borders, replete with minefields and guard posts.
Pyongyang previously insisted that not a single case of the virus had been recorded in the North despite the illness sweeping the globe, and the country's borders remain closed.
The patient was "was put under strict quarantine", as would any close contacts, KCNA said.
It was a "dangerous situation … that may lead to a deadly and destructive disaster", the media outlet said.
Mr Kim was quoted as saying "the vicious virus could be said to have entered the country" and officials took the "pre-emptive measure of totally blocking Kaesong City" on Friday.
North Korea closed its borders in late January as the virus spread in neighbouring China.
It imposed tough restrictions that put thousands of people into isolation, but analysts say the state is unlikely to have avoided the contagion.
China and North Korea share a 1,400-kilometre border that is especially porous during the winter, when frozen rivers allow people to cross more easily in and out of the two countries.
Dozens of North Koreans smuggle black market goods across the border every day and analysts said they may have carried the virus into North Korea before the borders were closed.
"There's no question the coronavirus in the North is imported from China," said Go Myong-Hyun, an analyst at the Asan Institute for Policy Studies, noting the heavy border traffic and China's high number of cases.
But Pyongyang was singling out the case from the South to highlight defectors as "dangerous beings", Mr Go said, as the North ramps up pressure against Seoul.
South Korea is recording between 40 and 60 new infections a day, with most of them imported cases.
This month Mr Kim warned against a hasty relaxation of anti-coronavirus measures, indicating the country would keep its borders closed for the foreseeable future.