Authorities in China have effectively placed the entire inland city of Xi'an under lockdown to contain the spread of the coronavirus.

Health officials say there have been more than 250 cases in the city, located in Shaanxi Province, between December 9 and Thursday.

Xi'an has about 13 million citizens. All are subject to PCR tests, and barred from leaving their homes in principle, starting Thursday.

One person from each household is permitted to go out once every two days to buy food and other necessities. Flights linking the city with other parts of China are suspended.

China has adopted strict controls under its zero-tolerance coronavirus policy. Authorities are stepping up vigilance ahead of the start of the Beijing Winter Olympics in early February.