China has vowed to avoid a "second Wuhan" after the capital, Beijing, reported its highest daily infection counts since late March for two consecutive days.
"Beijing will not turn into a second Wuhan, spreading the virus to many cities all over the country and needing a lockdown," Zeng Guang, a senior expert with China's national health commission, told state media, referring to the city from which coronavirus emerged late last year.
Health officials have quickly ramped up measures to contain the outbreak in Beijing, ending a 56-day streak of zero cases.
Thirty-six infections were confirmed on both Saturday and Sunday, bringing the total to 79 cases over the last four days in the city of more than 20 million.
Cases erupting in Beijing are the latest reminder that the authorities in China continue to struggle with subsequent waves of coronavirus, even after instituting draconian lockdowns and quarantine measures.
It remains unclear how people fell ill in Beijing, as China has reported nearly no local transmissions in recent weeks, finding cases primarily via infections brought in by travellers arriving from abroad.
"Our preliminary assessment is that the virus came from overseas. We still can't determine how it got here. It might have been on contaminated seafood or meat, or spread from the feces of people inside the market," Yang Peng, an epidemiologist with Beijing's Centre for Disease Control and Prevention, told state media.
Yang also said that genetic sequencing of the virus indicated it could be linked to a strain from Europe.
Aggressive contact tracing has found infected people either shopped or worked in Beijing's Xinfadi market, the largest wholesale food market in Asia, or came in contact with someone who had been there.
Some restaurants have begun pulling salmon from their menus, and the fish has been removed from supermarket shelves.
Local authorities in other parts of China are also warning against non-essential travel to Beijing.
Screening measures, which had been relaxed in recent weeks, have been stepped up, while schools and sports venues have been ordered to close again. A plan to allow grades one to three to return to school this week has been scrapped.
To enter public places such as shops, banks and parks, people must show a "green" health code – a clear contagion risk profile – submit to temperature checks and register personal details, such as ID and mobile numbers, to aid contact tracing. Entering residential and office compounds means flashing compound passes.
Thousands of tons of meat and produce are sold daily at the sprawling Xinfadi market, which is about 20 times the size of the Wuhan seafood market in which coronavirus was initially detected last December.