Authorities in the central Chinese city of Wuhan on Sunday said they had completed citywide testing of more than 11 million people for Covid-19 after a resurgence of cases more than a year after the coronavirus first emerged there.

The tests -- which began on Tuesday -- provide "basically full coverage" of all residents in the city except for children under the age of six and students on their summer break, senior Wuhan official Li Tao told a press conference, according to the state-run Xinhua.

By Saturday, the city had recorded 37 locally transmitted Covid-19 cases and found 41 local asymptomatic carriers in the latest round of mass testing, Xinhua reported.

City officials announced last week that seven locally transmitted infections had been found among migrant workers in Wuhan, breaking a year-long streak without domestic cases after it squashed an initial outbreak with an unprecedented lockdown in early 2020.

Authorities said they quickly mobilised more than 28,000 health workers at around 2,800 sites for the testing campaign.

China brought domestic cases down to virtually zero after the coronavirus first emerged in the city in late 2019, allowing the economy to rebound and life to return largely to normal.

But the fresh outbreak has thrown that record into jeopardy, as the fast-spreading Delta variant reaches dozens of cities after infections among airport cleaners in Nanjing sparked a chain of cases that have been reported across the country.

China has since confined the residents of entire cities to their homes, cut domestic transport links and rolled out mass testing as it battles the outbreak, its largest in months.

Beijing has also tightened overseas travel restrictions for its citizens as part of efforts to contain the rising cases.

China's immigration authority on Wednesday announced it would stop issuing ordinary passports and other documents needed for exiting the country in "non-essential and non-emergency" cases.

That does not yet mean a blanket overseas travel ban for the Chinese public.