Qingdao in East China's Shandong province has begun a program of free nucleic acid testing of all residents after new COVID-19 cases were linked to a local pulmonary hospital.
The city's health authority said tests across the city were expected to be completed over a five-day span.
China's National Health Commission on Monday sent a working group to Qingdao to guide COVID-19 epidemic control.
As of 12 am on Monday, the northeastern coastal city had confirmed six COVID-19 cases and reported another six asymptomatic cases, the municipal health commission said.
Local experts said all the cases were linked to wards at Qingdao Pulmonary Hospital, where COVID-19 patients who have arrived from overseas are treated in isolation.
The new cases surfaced on Sunday morning when the health authority reported three asymptomatic cases, two of which were confirmed later.
Nine others were found positive for the coronavirus after the city isolated and tested all three patients' known contacts— a total of 132 people. The rest came out negative, so far.
There are 171 people who are believed to have had close contact with the patients' contacts. They have been isolated for further observation and their results of nucleic acid testing are negative, so far.
Also tested were 840 people who are believed to have general contact with the patients. Their results are negative, so far.
In addition, the city tested 162,601 medical workers, patients and attendants in hospitals citywide. Of those, 154,815 have been found negative, so far.
Mass testing at community level has been going on. A total of 114,221 people have undergone the test. Of those, 75,296 have been found negative, so far.