After months of denying that COVID-19 had crossed its sealed borders, North Korea has placed the city of Kaesong under a sudden lockdown.

The North’s Korean Central News Agency (KCNA) reported on Sunday that the measure was taken on Friday after a single suspected  infection had been identified in the city.

According to KCNA, the regime’s domestic propaganda outlet, a person who defected to South Korea three years ago somehow slipped back across the fortified border.

If that person is officially declared a virus patient, it would be the North’s first confirmed coronavirus case.

North Korea has steadfastly said it has no single virus case on its territory, a claim no outside experts take seriously.

Leader Kim Jong Un also declared a state of emergency in the Kaesong area during a Politburo meeting on Saturday, KCNA says.

KCNA did not specifically mention whether the individual had been tested, but said an “uncertain result was made from several medical check-ups of the secretion of that person’s upper respiratory organ and blood,” prompting officials to quarantine the person and investigate anyone he may have been in contact with.

North Korea has received thousands of coronavirus testing kits from Russia and other countries, and imposed strict border closures.

Until recently thousands of people in North Korea were quarantined, but restrictions had recently eased.

In recent weeks, North Korea has also stepped up a campaign criticising defectors to South Korea as “human scum” and calling for South Korea to crack down on groups that send propaganda messages and food aid into the North.

Kim ordered an investigation into the military units along the border where the suspected COVID-19 carrier was suspected of crossing to “administer a severe punishment and take necessary measures”.

-with AAP