Qingdao, Shandong province, is still on high alert as the city reported six new confirmed COVID-19 cases, all of which were known to be positive for the coronavirus but who had no symptoms, according to the city's health authority on Wednesday.

The city has reported 12 confirmed cases since Sunday. Four patients are in serious condition, Chen Wansheng, deputy director of the general office of the city government, said on Wednesday.

By 6 pm on Wednesday, the city had collected and tested over 8.8 million samples and released more than 5.4 million results, with no additional positive results besides the 12 cases, the health authority said.

Acceleration of the effort to test every resident is underway, it added. Sample collection work from the city's five urban districts was expected to be completed by the end of Wednesday.

During a work meeting on Tuesday evening, Qingdao Party Chief Wang Qingxian called for the whole city to maintain a "state of war" spirit to fight the virus.

Experts said that the epidemic in Qingdao, which re-emerged over the weekend, will not likely develop into a large-scale outbreak.

"The mass nucleic acid testing effort is playing a crucial role in helping us learn how the epidemic evolves," said Wu Zunyou, chief epidemiologist at the Chinese Center for Disease Control and Prevention, during an interview with China Central Television on Tuesday evening.

More accumulated information in the Qingdao case has shown that the latest epidemic originated in hospital and is unlikely to develop quickly like those in Beijing's Xinfadi market in June and the Xinjiang Uygur autonomous region in July, Wu said.

Wu mentioned one of the confirmed cases－a taxi driver who has drawn wide attention. He was asymptomatic at first and became a confirmed case on Tuesday.

According to Wu, a patient is expected to be infectious for one or two days before displaying symptoms, and for five days after that. The taxi driver was isolated in time, and he is not believed to be very infectious, Wu said.

The Beijing government suggested on Wednesday that people from Qingdao not come to the capital if it's not necessary, and those who have to come need to show a negative nucleic acid test result from the previous seven days.

Xu Hejian, spokesman for the Beijing government, said at a news conference on Wednesday that people who came to the capital from Qingdao should follow related community epidemic prevention and control measures after arrival, such as providing regular health condition reports.

Gao Xiaojun, spokesman for the capital's health authority, said Beijing had traced five close contacts of the confirmed COVID-19 cases in Qingdao by Tuesday, and the test results for all five were negative.