China is racing to contain its worst coronavirus outbreak in months as fresh cases were reported in two more parts of the country including the sprawling megacity of Chongqing.

The highly contagious Delta variant has already been confirmed in the capital Beijing and four other provinces, prompting mass testing and putting more than one million people under lockdown, AFP reported.

More than 200 infections nationwide have been linked to a Delta cluster in eastern Jiangsu province, where cleaners at an airport in the city of Nanjing first tested positive on July 20.

The outbreak is geographically the largest in several months, challenging China's aggressive containment efforts, which have relied on mass testing, lockdowns and swift contact tracing.