Urumqi City, in northwest China's Xinjiang Uygur Autonomous Region, has entered "wartime mode" on Saturday after the detection of 17 domestic infections since July 15.
State broadcaster CCTV cited unnamed officials as telling a press conference on Saturday that the city had suspended gatherings and ordered communities to restrict visits to other households.
Mass screening for the virus will begin in buildings that had reported new cases and eventually cover all of Urumqi, said local health commission chief Zhang Wei.
According to state media reports, an official says, "The whole city has entered a 'wartime state', and will suspend all kinds of group activities."
Xinjiang was one of the first regions to let students return to school in late March after authorities declared an end to the initial wave of the COVID-19 outbreak.
The Chinese mainland has a total of 83,644 COVID-19 cases, with 109 asymptomatic patients currently under medical observation. The death toll stands at 4,652, including 11 from the Hong Kong Special Administrative Region and seven from the Taiwan region.
Beijing recorded no new domestically transmitted cases for the 12th consecutive day on Friday. From June 11 to July 17, the Chinese capital reported a total of 335 locally transmitted cases, 190 of which have been discharged from hospitals.