Nanjing in East China's Jiangsu province, launched a second round of nucleic acid testing of its 9.3 million residents and travelers on Sunday morning after 57 samples tested positive for COVID-19 in the first round of citywide testing.

Yang Dasuo, deputy director of the Nanjing Health Commission, said a total of 57 positive samples were collected in the city. Four were in Lishui district, one in Jianye, one in Gaochun and 51 in Jiangning, where Lukou International Airport is located, in the first round of mass testing.

"Nanjing tested 9.209 million people from 8 pm Wednesday to 8 pm Saturday," he said at a press conference held on Sunday. "Everyone in Nanjing should cooperate with the second round of testing work, follow guidance from medical workers, go to sample collection centers at different times, and avoid lining up for too long."

The city has set up 1,452 sample collection centers and 76 sample-analyzing institutions to analyze a maximum of 3.5 million samples a day since Wednesday after a group of airport staff, mostly cleaning and ground services workers, tested positive for COVID-19.

On Sunday, Nanjing updated one village in its Lukou sub-district from a medium to high-risk area. Four other areas were newly classified as medium risk.

It now has two high-risk and 26 medium-risk areas, according to the National Government Service Platform on Sunday afternoon.