SEOUL — North Korea locked down the city of Kaesong near the border with South Korea after finding what could be the country's first official coronavirus case there, state media reported Sunday.

North Korea’s state-controlled Central News Agency announced “a critical situation in which the vicious virus could be said to have entered the country” after a suspected patient returned from South Korea by illegally crossing the border last week.

Coronavirus test results were described as “uncertain,” but the person was still put under quarantine while health officials launched an investigation into those who might have come in contact with the individual in Kaesong, the state media reported.

If confirmed, he or she would be North Korea’s first official coronavirus patient in a country that has remained “virus-free,” according to Pyongyang authorities.

North Korean leader Kim Jong Un convened a Politburo meeting Saturday to address “the dangerous situation in Kaesong City that may lead to a deadly and destructive disaster,” state media said.

Kim declared a state of emergency in the Kaesong area and discussed lockdown measures that were put in place late Friday, shortly after the suspected case was reported.

A coronavirus outbreak would pose a significant threat to North Korea’s limited health system, which lacks basic protective equipment and medical supplies. The isolated country, mindful of the threat, further sealed itself off from the outside world as the novel coronavirus spread to areas near its border with China.

As early as January, North Korea shut down cross-border travel with China and Russia, although doing so severely limits its business with those countries. It has restricted domestic travel and placed diplomats and foreigners under effective house arrest.

More than 1,100 people in the country have been tested for the coronavirus as of July 9, the World Health Organization’s representative to Pyongyang was quoted as saying in NK News, an outlet based in Seoul that focuses on North Korea. The representative said all tests came back negative. About 600 North Koreans were under quarantine.

International aid organizations have provided coronavirus test kits and personal protective equipment to North Korea.

Kim urged the Politburo to “face up to the reality of emergency” and ordered the virus responses be elevated, a shift to “the maximum emergency system,” the Central News Agency reported.

Officials also discussed the lax performance of the guards along the part of the border where the suspected virus carrier crossed into North Korea, the state media reported, and promised to “administer a severe punishment and take necessary measures.”

The Central News Agency said the person in question had defected to the South three years ago.

More than 33,000 North Koreans have fled to the South since the early 1990s to escape poverty and political oppression. But it is rare for the defectors who settled in the South to head back.

South Korea’s military said in a statement that its joint chiefs of staff were inspecting the relevant surveillance recordings. “Along with related entities, we have identified and are currently reviewing cases of certain individuals based on specific timing and whereabouts,” the military said.

Defection across the heavily armed border between North and South Korea is extremely difficult. It is the world’s most fortified frontier, with land mines and armed soldiers.