NANJING, July 29 (Xinhua) -- Nanjing, a mega-city with a population of more than 9.3 million in eastern China, launched the third round of all-inclusive nucleic acid testing campaign on Wednesday, according to the municipal COVID-19 prevention and control headquarters.

The citywide testing covers all residents, including those who are paying a short visit to the city.

Nanjing, capital of east China's Jiangsu Province, has seen a resurgence of locally transmitted COVID-19 cases since a number of airport workers tested positive for coronavirus on July 20.

The city now has classified four areas as high-risk for COVID-19 and 42 as medium-risk areas, according to the municipal COVID-19 prevention and control headquarters.

On Tuesday, the city's center for disease control and prevention said the virus strain behind the recent spike in COVID-19 cases has been identified as the highly infectious Delta variant.

Officials therefore have urged those queuing up for the test to wear masks, keep at least one meter apart and avoid chitchat.