Travel restrictions were placed on nearly half a million people near Beijing on Thursday as authorities rushed to contain a fresh outbreak of the coronavirus with a mass test-and-trace effort and lockdowns in parts of the Chinese capital.

Another 21 cases of the virus were reported in the past 24 hours in Beijing, the National Health Commission said, taking the total to 158 since a fresh cluster was detected last week after months of no confirmed local transmissions.

One case was also recorded in the neighbouring city of Tianjin and two more in Hebei province -- which surrounds Beijing -- prompting travel restrictions on Anxin county, home to nearly half a million people, banning most traffic going in and out of the area. Essential service vehicles are allowed into Anxin, about 150 kilometres (90 miles) from Beijing, while private and government cars can enter and leave only if they have permission, state media said.

Beijing is collecting around 400,000 samples a day for testing amid fears the new outbreak could trigger a second wave of infections in China, which had largely brought the contagion under control since it emerged in the central city of Wuhan late last year.

"I had wanted to get tested anyway, but my workplace said all mall staff must be tested," a 25-year-old shop assistant surnamed Pang told AFP as she lined up at the Workers' Stadium in central Beijing to be swabbed. "I don't really mind waiting, it's for the greater good and the benefit of society."

A chef from a nearby restaurant, who gave his surname as Wang, said he had been sent by his boss to get tested. "Anyway we haven't had many customers over the past few days, people are scared to go out as much now," Wang told AFP.

'Under control'

The latest outbreak started in Beijing's sprawling Xinfadi wholesale market, which supplies more than 70 percent of the city's fresh produce. Some 30 residential compounds have been locked down. Wu Zunyou, chief epidemiology expert at the Chinese Center for Disease Control and Prevention, said a high number of infections had broken out among market traders selling seafood, beef and mutton.

However, he told reporters that the outbreak had peaked around June 13, and had now been "brought under control". "That doesn't mean there will be no patient report tomorrow," he warned. "This curve will continue for a period of time, and the number of cases will become less and less."

On Wednesday, an official at the Beijing Center for Disease Control and Prevention had said the number of people using the market would make the outbreak "hard to control". To prevent the virus spreading beyond Beijing, officials have urged residents to stay in the city and forbidden those in areas considered "medium and high risk" from leaving.

People who want to leave must be from a low-risk area and show they have tested negative for the virus within the last week. Eleven markets have been shuttered, thousands of food and beverage businesses disinfected, and schools closed again in the city.

Several bars and restaurants in the popular Sanlitun area were ordered to close on Thursday, with staff told to get tested for the virus. To secure the city's food supply, officials were setting up temporary markets, releasing more pork from state reserves and sourcing vegetables directly from growers, said commerce ministry spokesman Gao Feng.

Beijing's airports cancelled two-thirds of all flights on Wednesday and flight-tracking websites showed around 140 passenger flights had landed or departed so far on Thursday. The city normally handles more than 1,500 flights a day. China also reported four imported cases on Thursday among nationals returned from abroad.