NANJING -- The virus strain behind the latest clusters of COVID-19 cases in Nanjing, capital of East China's Jiangsu province, has been identified as the highly infectious Delta variant, local authorities said Tuesday.

The recent spike in infections in the city can be attributed to the special location of the outbreak and the highly contagious nature of the strain, said Ding Jie, vice director of the city's center for disease control and prevention, at a press conference.

Nanjing, a mega-city with a population of more than 9.3 million, has seen daily increases of locally transmitted COVID-19 cases since a few airport workers tested positive for coronavirus last week.

It has launched a second round of all-inclusive nucleic acid testing and urged residents not to leave the city unless necessary.