Chinese cities rolled out mass testing of millions of people and imposed fresh travel restrictions as health authorities battled Sunday to contain the country's most widespread coronavirus outbreak in months.

China on Sunday reported 75 new coronavirus cases with 53 local transmissions, with a cluster linked to an eastern airport now reported to have spread to over 20 cities and more than a dozen provinces.

The outbreak is geographically the largest to hit China in several months after the country's successes in largely snuffing out the pandemic within its borders last year.

That record has been thrown into jeopardy after the fast-spreading Delta variant broke out at Nanjing airport in eastern Jiangsu province in July.

Authorities have now conducted three rounds of testing on the city's 9.2 million residents and placed hundreds of thousands under lockdown, in an effort to curb an outbreak Beijing has blamed on the highly-contagious Delta variant and the peak tourist season.

Officials are now scrambling to track people nationwide who recently travelled from Nanjing or Zhangjiajie, a tourist city in Hunan province which has locked down all 1.5 million residents and shut all tourist attractions.

Fresh cases were reported Sunday in Hainan island -- another popular tourist destination -- as well as Ningxia and Shandong provinces, authorities said.

The country is also battling a separate rise in cases in the flood-ravaged city of Zhengzhou in Henan province after two cleaners at a hospital treating coronavirus patients coming from abroad tested positive.

27 locally transmitted cases have been detected, with authorities Sunday ordering mass testing of all 10 million residents. The head of the city's health commission has also been sacked.

And after reports that some people sickened in the latest cluster were vaccinated, health officials have said this was "normal" and stressed the importance of vaccination alongside strict measures.

"The Covid vaccine's protection against the Delta variant may have somewhat declined, but the current vaccine still has a good preventative and protective effect against the Delta variant," said Feng Zijian, virologist at the Chinese Center for Disease Control and Prevention.

More than 1.6 billion vaccine doses have so far been administered nationwide as of Friday, Beijing's National Health Commission (NHC) said. It does not provide figures on how many people have been fully vaccinated.