The northern Chinese city of Tianjin has ordered a second round of Covid-19 testing on all 14 million residents following the discovery of 97 cases of the Omicron variant during initial screenings which began on Sunday.
Residents must remain where they are until the results of all the nucleic acid tests are received, the official Xinhua News Agency said.
Xinhua said authorities have carried out almost 12 million tests so far, with 7.8 million samples returned. Infections were first reported on Saturday in the city that is only about an hour from Beijing, which is due to host the Winter Olympics from February 4.
The city's Covid-19 prevention and control office said all those who tested positive in the initial testing round were found to have the Omicron variant, of which China has so far only reported a handful of cases.
The source of the outbreak is still unknown and many who are spreading the strain may be doing so unwittingly because they show no symptoms.
Millions more are under lockdown in other Chinese cities under the strict "dynamic zero-case policy" that has allowed China to largely contain major outbreaks, although at considerable cost to the local economy.
Hong Kong has closed kindergartens and primary schools after infections were discovered among students, banned flights from the United States and seven other countries, and held 2,500 passengers on a cruise ship for coronavirus testing on Wednesday as the city attempted to stem an emerging Omicron outbreak.
The semiautonomous Chinese city has tightened pandemic-related restrictions in recent days after discovering the Omicron variant had spread beyond people arriving from overseas.
In total, China announced 166 new cases of Covid-19 in the 24 hours before Wednesday, including 33 in Tianjin and 118 in Henan province, but just eight in the city of Xi'an, home to the famed Terracotta Warrior statues and major industries, where a lockdown was imposed on December 23.
Authorities also released news of an inspection in Henan last week carried out by Vice Premier Sun Chunlan, who called for stepped-up measures to prevent the spread of both the Delta and Omicron variants.
Covid has spread to three cities in the province, including Yuzhou, Anyang, and the provincial capital Zhengzhou, prompting travel bans and various levels of lockdown.
Mr Sun was quoted as saying: "It is necessary to do a good job in guaranteeing the living needs of the people in the closed and controlled communities, ensure the basic medical needs of the people during the epidemic period, and ensure that the people's demands can be responded to and resolved in a timely manner."
Some Xi'an residents have complained about their inability to source food and other daily necessities.
China has sacked or otherwise punished a number of officials for dereliction of duty related to the pandemic response, including failing to prevent new infections.
On Wednesday, authorities in Anyang cited 61 government departments and 11 officials in violation. Punishments ranged from verbal warnings to dismissal, according to a city government notice.
Ensuring the Olympics are free of any outbreaks is a key concern for the ruling Communist Party.
Athletes, officials and journalists are operating in a closed-loop bubble to prevent them from coming into contact with the general public from the time they arrive in Beijing until their departure. If fully vaccinated, they will not have to undergo the standard 21-day quarantine.
In addition, the International Olympic Committee has issued a guidance document advising attendees to avoid crowded places such as bars and restaurants and avoid physical contact with others five days before departing.
Attendees are required to provide two negative PCR tests before they travel and will be tested twice daily while in China.
Participants should "absolutely not let your guard down" even after testing negative, the IOC said.
China has banned fans from outside the country from attending and it appears Beijing plans to distribute only a small number of tickets to carefully selected spectators.
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