Beijing, July 29 : Nanjing, a mega-city in China with a population of more than 9.3 million, has launched the third round of all-inclusive nucleic acid testing campaign in the wake of a fresh Covid-19 outbreak, authorities said on Thursday. The third citywide testing, launched on Wednesday, covers all residents, including those who are paying a short visit to Nanjing, reports Xinhua news agency. Nanjing, capital of Jiangsu province, has witnessed a resurgence of locally transmitted Covid-19 cases since a number of airport workers tested positive for the disease on July 20. The city now has classified four areas as high-risk and 42 others as medium-risk, according to the municipal Covid-19 prevention and control headquarters. On Tuesday, the city's centre for disease control and prevention said the virus strain behind the recent spike 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. /IANS