The World Health Organization said the number of cases in Beijing had risen to 100 over the course of the day, including likely to be reported Tuesday morning by the Chinese government.

The cluster is now the biggest the capital has had to deal with since February. The sudden resurgence of the virus is linked to the Xinfadi market, the biggest wholesale food market in Asia, and it has prompted city officials to lock down the market and several neighborhoods around it in a bid to prevent a broader second wave of infections.

At a state council meeting on Sunday, China's vice premier Sun Chunlan said the risk of the new outbreak spreading was "very high," and she called for decisive action to prevent that happening, according to the state-run news agency Xinhua.

"Beijing has entered an extraordinary period," city government spokesperson Xu Hejian said Sunday. City leaders have fired several officials in Fengtai District, where the new cluster originated, for misconduct in implementing epidemic control measures.

Chinese health workers carry out checks on residents, searching for anyone who recently visited or lives near the Xinfadi wholesale food market, at the closed Guangan Sport Center, June 14, 2020, in Beijing, China. Lintao Zhang/Getty

Beijing's outbreak has already spread to neighboring Hebei province, where three new cases were reported on Monday, and Liaoning province, where two cases were confirmed on Sunday. Both provincial health authorities said all the new cases were known to have been in close contact with patients from Beijing.

More than 21 neighborhoods in the capital have been put back under full lockdown, with 90,000 residents ordered to take COVID-19 tests. More than 8,000 staff from the Xinfadi market have been tested and are under quarantine and medical observation.