The Chinese mainland has reported more than 500 new COVID-19 infections for three consecutive days.

Data from the National Health Commission showed that the mainland reported 233 new confirmed local cases on Tuesday and 322 new asymptomatic infections, covering 19 provinces and regions.

Many cases were infections of the Omicron variant of the novel coronavirus which spreads more easily than other variants.

Beijing has urged residents not to leave the city unless necessary. It reported four new locally transmitted confirmed cases of COVID-19 and an asymptomatic carrier in the 24 hours to 4 pm Wednesday.

"With the pandemic still spreading quickly in many foreign nations and outbreaks also being reported in some domestic cities, Beijing is still facing dual risks from both domestic cities and foreign nations and regions," Pang Xinghuo, deputy director of the Beijing Center for Disease Control and Prevention told a news conference on Wednesday.

"The authorities should tighten measures on people returning to Beijing from other areas in the country and will implement strict, rapid and tight prevention and control measures and quickly carry out case investigation and screening of at-risk people to help effectively block and reduce the risk of the further spread of cases in Beijing."

Special measures will also be introduced to expand investigation, screening, management and control at airports, coach terminals, road checkpoints and other transportation hubs, hotels, guesthouses, schools and related units in Beijing, Pang said.

Three of the new infections in Beijing were in the capital's Haidian district and two in Chaoyang district.

Genome sequencing has indicated the cases in the current outbreak in Beijing involve an Omicron variant very similar to that found in cases in an unspecified province.

Meanwhile, Jilin province saw the biggest increase of confirmed cases of COVID-19 on the Chinese mainland on Tuesday.

The province reported 94 new local confirmed cases on Tuesday, including 73 in the city of Jilin, 12 in Changchun, the provincial capital, and nine in Yanbian Korean autonomous prefecture.

Another 80 asymptomatic carriers were detected, including 65 in Jilin city, 10 in Changchun, two in Yanbian, two in Meihekou and one in Songyuan.

The strain of novel coronavirus found in the recent COVID-19 outbreak in Jilin city was the Omicron variant, an official said on Wednesday.

"The number of confirmed cases is increasing rapidly," Sun Dianwei, director of the city's center for disease control and prevention, told a news conference on Wednesday afternoon. "The epidemic is on the rise and there has been community transmission."

The city started its fourth round of mass testing at noon on Wednesday. It was expected to be finished by 6 pm.

"Two designated hospitals, including Jilin Hospital for Infectious Diseases and the Affiliated Hospital of Jilin Medical University can provide 552 beds for infected patients," Luo said. "Another three makeshift hospitals with around 1,200 beds are being prepared."

Positive cases related to the Jilin city outbreak were also reported in Changchun.

"The first case traveled to Jilin city several times before being diagnosed as COVID-19," Jiang Hongbo, deputy director of the Changchun health commission, said at a news conference on Wednesday.

Zhang Xingguo, director of the city's center for disease control and prevention, said: "The epidemic in Changchun is on the rise, and the number of cases may increase. There are also some infected students in the schools."

Separately, Shanghai reported four new locally transmitted COVID-19 cases and 15 asymptomatic cases on Wednesday.

According to the municipal health authority, one locally transmitted case from Pudong New Area, aged 8, tested positive when she went to a hospital with fever symptoms.

The primary school where she studies was subsequently categorized as a medium-risk area for the virus' spread. The other cases were close contacts of confirmed cases announced in the past days who tested positive during quarantine.

Zhang Yangfei and Han Junhong contributed to this story.