Beijing has imposed a lockdown of 1.7 million people in part of the Chinese capital as officials race to prevent a COVID-19 resurgence in the country’s northern region from seeping into its most important city.

Daxing district in southern Beijing, where its new airport is located, has been sealed off from the rest of the country after six infections were found there. The total number of cases in Beijing stood at 23 on Wednesday, while over a thousand infections have been found nationwide since early January, mostly in China’s vast rural northern provinces.

While the number of cases is small compared to outbreaks in western countries, the flareup — fueled by an unusually cold winter — is China’s biggest coronavirus challenge since the Wuhan crisis a year ago given its potential to spread to the capital of over 20 million people, China’s cultural and political center.