Qingdao, Shandong province, reported six new COVID-19 cases on Tuesday, all of which were the six people known be positive for the coronavirus but who had no symptoms, city information officials said at a news conference on Wednesday morning.

So far, the city has reported 12 confirmed cases, including what was said to be an extremely dangerous infection of a 38-year-old along with three other serious cases, said Chen Wansheng, deputy director of the general office of the city government on Wednesday.

Members of a medical team consisting of eight experts from across the country specializing in respiration, infection, intensive care and traditional Chinese medicine have been working furiously on the front line to save the patients, Chen said.

Shandong also sent six experts and three nurses to designated hospitals to support treatments.

At the same time, an effort to test every resident is underway.

As of 9:30 am on Wednesday, the city had collected and tested more than 7.5 million samples and released more than 4 million results, all negative, officials said.

Vigorous efforts are being made to continue trace the patients' close contacts. "The city has isolated and tested a total of 520 contacts," Chen said.

All 388 staff members, patients and their caregivers in the city's pulmonary hospitals, to which the 12 cases are closely linked, have had their contacts traced," Chen said.