A view of a deserted road after the city has locked down in north China's Tianjin Municipality, on Monday. AP

A third Chinese city has locked down its residents because of a COVID-19 outbreak, raising the number confined to their homes in China to about 20 million people.

It wasn't clear how long the lockdown of Anyang city, home to 5.5 million, would last as a notice said it was being done to facilitate mass testing but did not indicate if it would end when the testing is completed.

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Five million residents of a central Chinese city started home confinement Tuesday in a new lockdown to curb the spread of the highly transmissible Omicron variant of the coronavirus, according to state media.

Beijing is on high alert for new outbreaks as it prepares to host the Winter Olympics next month, sticking to a "zero-Covid" strategy of targeted lockdowns, border restrictions and lengthy quarantines.

Residents wearing face masks wait for the coronavirus test in north China's Tianjin Municipality, on Monday. AP

Another 13 million people are locked down in the city of Xi’an and 1.1 million in Yuzhou.

While those measures have kept the number of new cases far lower than virus hotspots in the United States and Europe, Beijing is currently battling local flare-ups in multiple cities.

The fast-spreading Omicron strain presents a fresh challenge, with two cases in Anyang -- the newly locked down city in Henan province -- linked to a growing infection cluster in the northern metropolis of Tianjin, some 400 kilometres (250 miles) away.

The lockdown of Anyang followed the confirmation of two cases of omicron on Monday that are believed to be linked to two other cases found Saturday in the city of Tianjin. It appears to be the first time omicron has spread in China beyond people who arrived from abroad and their immediate contacts.

About 2,000 people have been infected in Xi'an in what is by far the largest outbreak in China.

Residents are not allowed to leave their homes, non-essential vehicles are banned from streets and stores have been ordered shut except for those selling necessities, according to a city notice shared by state media late Monday.

Xi'an and Yuzhou are both battling the delta variant and neither has reported any omicron cases. About 2,000 people have been infected in Xi'an, an ancient capital that is home to the Terracotta Warrior ruins, in what is by far the largest outbreak in China.

Authorities in Anyang announced the lockdown late on Monday, issuing a notice ordering residents not to leave their homes or drive cars on the roads, state news agency Xinhua reported.

All non-essential businesses have been closed and a mass-testing drive has been launched "to respond to the severe epidemic control situation and strictly prevent the spread of the Omicron virus outbreak", Xinhua reported.