Chinese authorities have confined Chengdu, a southwestern city of 21 million people, following a surge in COVID-19 cases.

The neighbors were ordered to stay at home and around 70% of the flights to and from the city were suspended. Chengdu is an important hub of air connections in Sichuan Province, as well as an economic and government center.

The start of the new school year was postponed, although public transport was still in operation and citizens could leave the city if they showed a special need.

Under rules announced Thursday, only one member of each family who could show a negative test from the previous 24 hours could leave each day to buy essentials.

It was not announced when the lockdown would be lifted.

Similar measures have confined millions of people to their homes in the northeastern city of Dalian, as well as in Shijiazhuang, capital of Hebei province and bordering the capital, Beijing.

Chengdu has reported about 1,000 infections in the new outbreak, but no deaths. The strict measures are framed in China’s rigid “zero COVID” policy, which has taken a heavy toll on the economy, with quarantines, business closures and mass testing requirements.

China says the measures are necessary to prevent further spread of the virus, first detected in the central Chinese city of Wuhan in late 2019. The fear of being confined or being sent to a quarantine center for contact with an infected person has greatly limited the work, consumption and travel habits of the population.