Four areas in the city of Nanjing, east China's Jiangsu Province, where nine airport workers tested positive for COVID-19 on Tuesday, heightened their COVID-19 risk levels from low to medium on Wednesday, according to the municipal government.

All four areas are located in the district of Jiangning where the airport is located. A mass nucleic acid testing campaign is being conducted across the district.

Personnel movement restrictions have been imposed and residents have been asked not to leave the city unless necessary. 

Those who plan to fly out of Nanjing must hold a negative nucleic acid test certificate issued maximum of 48 hours before their departure.

According to VariFlight, a China-based civil-aviation data service provider, at least 480 flights have been canceled as of 8:40 a.m. on Wednesday at Nanjing Lukou International Airport.

(With input from Xinhua)