In Beijing, 17 new confirmed cases were reported, up from 11 a day earlier and the most since June 20.
Anxin, which forms a part of the Xiongan New Area launched in 2017 to provide living space for Beijing, has also instructed residents to stay indoors, with "the epidemic prevention and control situation now extremely serious", according to state media.
Officials announced on Sunday that Anxin would be "fully enclosed and controlled".
Only essential workers are allowed to leave their homes, while one member of a household is allowed to go out once a day to shop for necessities.
No non-residents residents will be allowed to enter buildings, communities or villages.
The containment measures in Anxin, located 140 km from Beijing, are more severe than in the capital itself where the cluster has grown to 311 people since it was first detected on June 12.
As the number of people killed by the coronavirus passes 500,000 worldwide, China isn't taking any chances of a second wave gripping the country - despite having largely contained its outbreak in recent months.
The National Health Commission (NHC) said in a statement that five of the new COVID-19 cases were so-called imported infections involving travellers from overseas, compared with three such cases reported a day earlier.
While this is a small number compared with the thousands of daily cases in the US or South America, China reacted quickly to contain any spread.
Before the recent spike, the Chinese capital had gone 57 days without a locally-transmitted case.
Speaking to CNN in May, China's top respiratory expert Dr. Zhong Nanshan warned that China still faced the "big challenge" of a potential return of the virus, and authorities should not be complacent.
The country has "flattened the curve". The seven local infections were all in the capital city of Beijing.