North Korea has reported what it describes as the country's first suspected case of coronavirus.
State media said a person who defected to South Korea three years ago last week returned across the demarcation line with Covid-19 symptoms.
Leader Kim Jong-un held an emergency meeting with top officials, imposing a lockdown in the border city of Kaesong.
North Korea, a secretive state, had previously not reported any virus cases - but analysts said this was unlikely.
"An emergency event happened in Kaesong city where a runaway who went to the south three years ago - a person who is suspected to have been infected with the vicious virus - returned on 19 July after illegally crossing the demarcation line," news agency KCNA said.
At Saturday's politburo meeting, Mr Kim was said to have ordered a "maximum emergency system" to contain the virus.
Mr Kim also launched an investigation into how the person had managed to cross the heavily fortified border, KCNA added, warning those responsible that "a severe punishment" would be administered.
On Sunday, a South Korean military official said there were "high chances" that an individual had illegally crossed into the North and that an investigation had been launched, according to Yonhap news agency.
North Korea closed its borders and put thousands of people in isolation six months ago, as the virus swept across the globe.
Earlier this month, Mr Kim hailed his country's "shining success" in dealing with Covid-19.
What does this mean for North Korea?
By Alistair Coleman, BBC Monitoring
There have been rumours of Covid-19 cases in North Korea for months, but the country's tightly-controlled society and state media have made them impossible to confirm. So the announcement that there's a suspected case in Kaesong comes as something of a surprise.
The circumstances of this one case seem unusual, too. It is exceedingly rare for individuals to defect over the hugely fortified border with South Korea (they normally flee over the northern border to China), let alone re-defect via that route several years later.
But South Korean authorities say that there may be evidence that this unlikely situation has come to pass. Whether the person involved had Covid-19 at the time is another question entirely.
It remains to be seen how Supreme Leader Kim Jong-un handles this.
Outsiders fear the country's impoverished health system will struggle to contain Covid-19. However, the country's poor road and rail infrastructure, alongside existing authoritarian restrictions on travel between cities for most citizens, could actually work to the North Korean government's advantage.
Such is the stranglehold Pyongyang has over information in the world's least-free media environment, it's likely we'll never be able to find out what's really happened.