China reported 76 new COVID-19 cases on July 25, the highest since the end of January amid a surge of local infections in the eastern city of Nanjing, as it starts a second round of mass testing to contain the outbreak.

China has taken a zero-tolerance approach to COVID cases, quickly tracing and testing wide swathes of its population to prevent the spread of the virus.

Local infections accounted for 40 of the new cases, compared with only five a day earlier, the National Health Commission said in a statement on Monday.

Thirty-nine of the local cases were reported in the eastern province of Jiangsu, of which Nanjing is the capital, and one in the northeastern province of Liaoning, it said.

The number of new asymptomatic cases - which China does not classify as confirmed cases - rose to 24 from 17 cases a day earlier.

Among the symptomless cases, four were local infections - one in Jiangsu, one in Guangdong, one in Jiangsu's neighbouring province of Anhui, and one in Sichuan province.

Many of the positive cases in the first round of testing Nanjing launched last week were in an area close to the Lukou International Airport.

Nanjing was conducting a second round of nucleic acid testing of its 9.3 million residents on Sunday, the official China Daily said on Monday.

The city raised its COVID risk level in one area to high, while changing other areas to a medium risk level, the paper added. The move comes after it suspended its subway line and took other measures to control the new cluster.

Total confirmed COVID-19 cases in mainland China stand at 92,605, and the death toll remains at 4,636.