China was set to conduct a mass-screening campaign in the 9 million-strong metropolis of Qingdao on Monday after six cases of COVID-19 were discovered, health authorities said.
In contrast to other parts of the world where cases are soaring, China has for several months managed to contain the epidemic with strict controls, the widespread wearing of masks, containment measures and contact-tracing applications.
Nationally, only a few new patients have been recorded each day, almost all of them Chinese who have returned from abroad and are placed in quarantine upon arrival.
But the discovery on Sunday of six positive cases of coronavirus and six asymptomatic cases in the coastal city of Qingdao fuelled fears of a resurgence of the virus.
City authorities have announced a testing campaign in five districts of Qingdao "within three days" and the whole city "within five days". However, the authorities did not specify exactly how many people would be tested.
The source of the cases is not yet known, but local health authorities have said all the infected people appear to have a link to a hospital in the city that treats COVID-19 patients.
The discovery has led to the initial screening of around 143,000 people in an attempt to identify contact cases.
In May, China tested the entire city of Wuhan - believed to be the epicentre of the pandemic and home to around 11 million people - over a 10-day period.
The new cases come after the country celebrated its Golden Week Holiday, which saw millions travel across the country.
Every summer, many tourists flock to Qingdao, which is famous for its "Tsingtao" beer, for a Beer Festival that is the largest of its kind in China. Thousands of beer-lovers gathered in August for this year's event.
To date, China has recorded 85,578 cases of coronavirus and 4,634 fatalities.