Hundreds of thousands more people were ordered to stay home in northern China Tuesday, joining millions under strict lockdown as authorities raced to contain a surge in Covid cases that have reached a 21-month high.

China -- where the virus emerged two years ago -- has followed a "zero-Covid" strategy of tight border restrictions, lengthy quarantines and targeted lockdowns as Beijing prepares to welcome thousands of overseas visitors to February's Winter Olympics.

But authorities have faced a resurgent virus in recent weeks, reporting 209 infections on Tuesday -- the highest single-day tally since March last year, when the pandemic raged through the central city of Wuhan.

The surge -- while low in comparison to rampant cases in Europe and the United States -- has prompted authorities to impose what they have called the "strictest" possible curbs in the northern city of Xi'an, whose 13 million residents are entering a sixth day of home confinement.

Chinese authorities have faced a resurgent virus in recent weeks, reporting 209 infections on Tuesday -- the highest single-day tally since March last year, when the pandemic raged across the country
Chinese authorities have faced a resurgent virus in recent weeks, reporting 209 infections on Tuesday -- the highest single-day tally since March last year, when the pandemic raged across the country AFP / STR

Nearby cities have also logged cases linked to the flare-up, with Yan'an -- about 300 kilometres (186 miles) from Xi'an -- Tuesday shuttering businesses and ordering hundreds of thousands of people in one district to stay indoors.

The Xi'an lockdown is the most sweeping in China since the similarly-sized Wuhan was sealed off.

The city has set up over 4,400 sampling sites and deployed more than 100,000 people to handle the latest round of testing, according to state broadcaster CCTV. Footage showed masked residents queueing to be tested in streets and sports centres.

Many Xi'an residents have taken to social media to call for help in acquiring food and other essentials
Many Xi'an residents have taken to social media to call for help in acquiring food and other essentials AFP / Noel Celis

But banned from driving and limited to sending out one household member every three days to buy groceries, many Xi'an residents have taken to social media to call for help in acquiring food and other essentials.

"I'm about to be starved to death," wrote one person on the Weibo site.

"There's no food, my housing compound won't let me out, and I'm about to run out of instant noodles ... please help!"

"I don't want to hear any more news about how everything is fine," said another.

"So what if supplies are so abundant -- they're useless if you don't actually give them to people."

Authorities have insisted that supplies remain stable as they maintain strict controls on movement into and out of Xi'an.

Students have also been banned from leaving university dormitories unless necessary, CCTV reported.