China's worst coronavirus outbreak in months has spread to two more parts of the country, health authorities said Saturday, including a sprawling megacity of 31 million people.

The National Health Commission reported 55 new cases in areas including Fujian province and Chongqing municipality, in addition to the four provinces and Beijing where outbreaks of the Delta variant had already been reported.

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

Nanjing city authorities ordered all tourist attractions and cultural venues not to open on Saturday, prompted by the spike in domestic transmissions.

Hundreds of thousands have already been locked down in Jiangsu province, while Nanjing has tested all 9.2 million residents twice.

The tourist city of Zhangjiajie in Hunan province, where a handful of cases attended a single theatre performance, locked down all 1.5 million residents and shut all tourist attractions Friday, according to an official notice.

Famed for its striking rock formations, the city is where part of the "Avatar" blockbuster was filmed.

In Beijing's Changping district, where two locally transmitted cases have been found, 41,000 people in nine housing communities were placed under lockdown Thursday.

China has previously boasted of its success in snuffing out the pandemic within its borders after imposing the world's first virus lockdown in early 2020 as Covid-19 seeped out of Wuhan in the centre of the country.

But an outbreak this month driven by the fast-spreading Delta variant has thrown that record into jeopardy.