Authorities have raced to contain the outbreak linked to the largest wholesale food market in Beijing after the first case was announced on June 11, leading to a partial lockdown of the city.

"The Beijing epidemic directly linked to Xinfadi [market] is basically under control, but at the same time we have discovered household and workplace cluster infections and cases of community transmission," said Beijing municipal government spokesman Xu Hejian at a briefing.

"The prevention and control situation remains complicated, we cannot lower our guard in the slightest."

Officials found that 253 out of 256 Beijing cases were linked to Xinfadi in the south of the city, while contact tracing for the remaining three was still ongoing.

The city announced seven new cases on Wednesday, with the rate of infection slowing since the start of this week.

"This sends a very positive signal, and proves that the prevention and control measures taken lately, as well as lockdown of residential compounds, are effective," said Lei Haichao, head of the Beijing health commission.

"The combination of fever clinics, discovering (cases) from close contacts, and mass nucleic acid testing have played an important role in early detection."

The city increased its daily nucleic acid sample testing capacity to 300,000 from 100,000 per day in early June, said Lei, adding that 137 cases -- just over half -- were found through screening.

Nearly three million people have been tested.

Many cases have been low-income migrant workers at Xinfadi and nearby restaurants. The market and dozens of residential compounds have been under lockdown since June 13.

Beijing has mass-tested wholesale market workers, restaurant workers, residents of medium and high-risk neighborhoods and delivery couriers over the past week.

The cities of Wuhan, where the initial COVID outbreak emerged late last year, and Mudanjiang have also seen similar mass testing campaigns to root out undiscovered asymptomatic cases.

Experts warned that smaller, recurrent outbreaks were likely in future.

"There may be an increase in cases in the winter or next spring, but I don't think the outbreak will be as big as the first wave of the pandemic," said Zhong Nanshan, a leading respiratory expert who spearheaded China's response to the SARS pandemic, on Wednesday.