More than half a million people were placed under lockdown in Beijing on Monday as the government imposed strict measures to stamp out a handful of Covid-19 cases.

China has largely brought the virus under control, but is tackling a number of local infections with lockdowns and mass testing.

Authorities are keen to stem any outbreak in the capital -- home to over 20 million people -- particularly ahead of a week-long national holiday next month.

All rural villages in Shunyi district on the outskirts of Beijing are locked down until a fresh round of mass testing has been completed, district official Zhi Xianwei said at a press briefing Monday. 

The move means around 518,000 residents will not be permitted to leave their villages until they have undergone testing. 

Officials also said locals would be under "closed management", suggesting they will be barred from leaving their residential compounds.

The district, which has a total population of 1.2 million, is home to many rural migrant workers. 

Zhi said officials would "knock on doors... all migrants will be classified and accounted for."

Experts have not yet identified the source of the outbreak, but one asymptomatic case confirmed on Sunday was a driver for an online ride-hailing service, and the government has suspended all taxi-hailing services in the district.

Beijing reported one new case on Monday and has recorded 32 local transmissions of Covid-19 in total since December 19, mostly in Shunyi.

Shunyi residents already completed a round of mass testing in late December, when six villages and an industrial park were also locked down and delivery couriers were banned from entering residential estates. 

Although the latest outbreak is modest compared to Beijing's last cluster in June, when more than 300 were infected, officials are anxious to contain the spread of the virus before the country's massive annual migration for the Lunar New Year holiday next month.

The Beijing government has cancelled large-scale events and urged residents not to leave the city over the holidays to minimise the risk of infection.