SEOUL--North Korea’s nominal head of state inspected the locked down border town Kaesong after a defector suspected of having the coronavirus returned from South Korea, said state media, signaling serious concerns about cross-border contamination.

The isolated country has claimed it has no domestic virus cases and has imposed strict quarantine measures and screening in Kaesong, just north of the border with the South, where the suspected coronavirus infection was reported.

Seoul officials said a 24-year-old man from Kaesong defected to South Korea in 2017 and had returned to the North this month after facing a sexual assault investigation in the South. He crawled through a drainpipe and swam across the river to cross the border.

North Korea has not formally confirmed the man tested positive for the virus but said he was showing symptoms. South Korean health officials said there was no sign that he was infected before he crossed the border, and at least two people who were in close contact with him have tested negative.

Choe Ryong Hae, president of the Presidium of the Supreme People’s Assembly, has visited the sealed city and nearby districts to check on the virus prevention measures, the official KCNA news agency said on Thursday.

“He emphasized that disinfection and quarantine must be strictly carried out as supplies including food and medicine are provided to Kaesong citizens,” KCNA said.

The Rodong Sinmun, a ruling Workers’ Party mouthpiece, also called for thorough measures to prevent the virus, warning against carelessness and inertia.

“Even though there has not been a single infection in our country, a moment of inattention could cause a fatal crisis that we can’t imagine or make up for,” the newspaper wrote on Thursday.