BEIJING (AFP) - China on Wednesday (Aug 4) reported its highest daily number of local coronavirus cases in months as mass testing and contact tracing campaigns uncovered a trail of infections linked to the Delta variant.

The health authorities reported 71 domestic cases on Wednesday, the highest since January, as China battles its largest outbreak in months by testing entire cities and locking down millions.

The official results of those tests have revealed a low caseload despite the outbreak spreading to dozens of major cities.

Beijing had previously boasted of its success in crushing Covid-19, allowing the economy to rebound and normal life to return while swathes of the globe struggled to douse a pandemic that has killed more than four million people worldwide.

But the latest outbreak is threatening that record, with nearly 500 domestic cases reported since mid-July when a cluster among airport cleaners in Nanjing, Jiangsu province, was found.

Most cities have yet to impose strict citywide lockdowns but a few have limited some movements.

Wuhan, where the virus first emerged in 2019, reported its first local infections in over a year this week and said on Tuesday it was "swiftly launching" testing of all 11 million residents.

Long lines of residents waited at outdoor testing stations in the summer heat on Tuesday, fanning themselves with paper forms while workers in hazmat suits took throat samples.

Meanwhile, Nanjing has tested its 9.2 million residents three times after shutting down gyms and cinemas and closing off residential compounds.

The tourist destination of Zhangjiajie in central Hunan province, where infected travellers who had been in Nanjing attended a theatre performance, abruptly announced on Tuesday that no one would be allowed to exit the city after it emerged as an infection hot spot.

Including symptomatic patients arriving from abroad, the number of new cases stood at 96 for Aug 3, up from 90 a day earlier.

China reported 27 new asymptomatic coronavirus cases, which it does not classify as confirmed cases, compared with 41 a day earlier. No new deaths were reported.

Mainland China had recorded 93,289 confirmed cases by Tuesday. The cumulative death toll was unchanged at 4,636.