The Chinese city of Qingdao said on Monday it will conduct coronavirus tests for its entire population of more than 9-million after reporting new cases that appeared to be linked to a hospital treating imported infections.

Daily Covid-19 infections in mainland China have fallen drastically since the early days of the outbreak, which first emerged in the city of Wuhan. China has reported no new domestically transmitted cases since early August, but it remains on high alert.

Qingdao reported a total of six new Covid-19 cases and six asymptomatic cases as of 11pm on Sunday, including three asymptomatic cases announced earlier in the day. All of the cases were linked to the Qingdao Chest Hospital.

Testing is being accelerated and will cover the city in five days, it said.

Qingdao's mass testing campaign is not China's first. Wuhan city tested its entire population, and mass schemes involving several million samples have also been conducted in Beijing and Urumqi.

After the latest infections, an investment and trade expo organised by the Shanghai Cooperation Organisation and scheduled for the weekend of October 16-18 in Qingdao has been postponed, China's state TV reported on Monday.

The National Health Commission's (NHC) tally of 21 confirmed Covid-19 cases in mainland China for October 11 was published after Qingdao's announcement, but did not include any confirmed infections in the city. It was not immediately clear why.

The number of new nationwide asymptomatic cases, which China does not count as confirmed cases, rose to 32 from 23 a day earlier, the NHC said. It did not offer a breakdown on where the new asymptomatic cases were reported, though it said 29 of these cases were imported.

Qingdao said it has locked down the Qingdao Chest Hospital as well as the emergency department of its central hospital. Buildings housing infected individuals have also been locked down as part of the city's virus containment measures.

Total confirmed Covid-19 cases in mainland China now stand at 85,578. The death toll remains at 4,634.

Qingdao's new infections occurred shortly after China completed its Golden Week holiday, during which millions of people travelled domestically.