China has sent more than 10,000 health workers from across the country to Shanghai, including 2,000 military medical staff, as it struggles to stamp out a rapidly spreading COVID-19 outbreak in China's largest city.

The city was conducting a mass testing of its 25 million residents Monday as what was supposed to be a two-phase lockdown entered its second week. While many factories and financial firms have been able to keep operating by isolating their employees, concern was growing about the potential economic impact of an extended lockdown in China's financial capital and a major shipping and manufacturing center.

The highly contagious omicron BA.2 form of the virus is testing China's ability to maintain its zero-COVID approach, which aims to stop outbreaks from spreading by isolating everyone who tests positive, whether they have symptoms or not. Shanghai has converted an exhibition hall and other facilities into massive isolation centers where people with mild or no symptoms are housed in a sea of beds separated by temporary partitions.

China on Monday reported more than 13,000 new cases nationwide in the previous 24 hours, of which nearly 12,000 were asymptomatic. About 9,000 of the cases were in Shanghai. The other large outbreak is in northeastern China's Jilin province, where new cases topped 3,500.

The English-language China Daily newspaper said that nearly 15,000 medical workers from neighboring Jiangsu and Zhejiang provinces left for Shanghai early Monday from their hospitals by bus. More than 2,000 personnel from the army, navy and joint logistics support force arrived on Sunday, a Chinese military newspaper said.

At least four other provinces have also dispatched doctors, nurses and other medical workers to Shanghai, the state-owned China Daily said.

While most shops and other businesses in Shanghai are shut down, major manufacturers including automakers General Motors Co. and Volkswagen AG say their factories still are operating. VW has reduced production because of a disruption in supplies of parts.

Businesses that are operating are enforcing “closed loop” strategies that isolate employees from the outside. Thousands of stock traders and other people in financial industries are sleeping in their offices, according to the Daily Economic News newspaper.

Three out of five foreign companies with operations in Shanghai say they have cut this year’s sales forecasts, according to a survey conducted last week by the American Chamber of Commerce in Shanghai and the American Chamber of Commerce in China. One-third of the 120 companies that responded to the survey said they have delayed investments.