China has put a city near the North Korea border under lockdown due to an increase in coronavirus infections, raising more questions about an outbreak in the isolated country.

Chinese authorities banned all nonessential transportation in the city of Shulan in the northeastern province of Jilin, while residential compounds and villages were closed, the official China Central Television reported Sunday. Students who had already returned to schools were required to study from home. The city raised its virus threat alert level to high risk from medium, Jilin province said.

North Korea shut its borders in January when cases surged in China, and has yet to confirm any COVID-19 infections. Yet the U.S. military said it suspects North Korea has cases, and Kim Jong Un’s regime has accepted help from other nations to fight the virus.