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

Four positive samples were collected in the city's Lishui district, one in Jianye, one in Gaochun and 51 in Jiangning, where the Lukou International Airport is located, said Yang Dasuo, deputy director of Nanjing's health commission.

"From 8 pm Wednesday to 8 pm Saturday, Nanjing completed nucleic acid testing of 9.209 million residents," he told a news conference on Sunday. "We hope everyone will cooperate with the second round of testing, follow guidance from medical workers, go to sample collection centers at different times, and avoid lining up for too long."

Nanjing has set up 1,452 sample collection centers and 76 institutions that have analyzed 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 the novel coronavirus.

The Jiangsu Provincial Health Commission said 37 locally transmitted cases were undergoing treatment at a designated hospital on Sunday morning.

The province has placed 1,318 contacts of infected patients and asymptomatic carriers under medical observation.

Nanjing raised the risk level in a village in its Lukou subdistrict to high on Sunday, up from medium, and classified four other areas as new medium-risk locations.

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

Ding Jie, deputy director of the Nanjing center for disease control and prevention, said the launch of the second round of testing was needed to contain the virus.

"Local residents could possibly get infected as the city has several medium- and high-risk areas," she said. "Some patients may never show, or at least may not have shown, symptoms after they are infected. Therefore, large-scale testing to find potential risks is necessary."

The results of nucleic acid tests could be affected by many factors, such as different stages of the disease or the different situations of patients, Ding added, and infections might not be detected if samples contained insufficient amounts of the virus.

To guarantee that all those leaving the city are in possession of a negative nucleic acid test report no more than 48 hours old, Nanjing has set up 68 checkpoints on roads around the city.

Neighboring cities have all strengthened pandemic responses, including quarantining people who have been to Lukou International Airport and asking those who have visited Nanjing to take extra nucleic acid tests.

Mianyang in Sichuan province, Zhongshan in Guangdong province, Shenyang in Liaoning province, and Ma'anshan and Wuhu in Anhui province have all reported locally transmitted cases related to people who had been to Nanjing.