Tianjin emerged as a new area of concern after more than 20 Covid cases were reported there in the last few days, most of them imported from abroad, according to the National Health Commission.

They include at least two cases of the Omicron variant, as well as 15 infections among elementary and middle school students, according to various state media reports.

The city near the capital Beijing launched its mass testing early Sunday, advising residents to stay at or near home to be available for the community-level nucleic-acid screening.

So far, however, no larger lockdown order was seen.

Chinese authorities have already been struggling with a larger outbreak centred on the northwestern city of Xi'an that has sparked questioning of the country's zero-tolerance policy of strict lockdowns and immediate mass testing to curb outbreaks.

China, where the coronavirus was first detected in 2019, has thus far reported only a handful of Omicron cases.

Tianjin residents have been told that until they obtain a negative test result, they will not receive a "green" code on smartphone Covid-tracing apps that nearly all people in China are now required to present when using public transport and in other situations.

Tianjin is a major port city about 150 kilometres (90 miles) southeast of Beijing.

Xi'an, historic home to China's famed Terracotta Warriors, was locked down last month, forcing its 13 million residents indoors. New case numbers there have slowed in recent days.

Officials have faced complaints from Xi'an residents over chaotic handling of the lockdown, including poor access to food and daily essentials, and viral cases such as a miscarriage suffered by an eight-month pregnant woman who was refused entry to a hospital without a Covid test.

China's government has touted its initial strict response to Covid, which has largely brought the pandemic under control within its borders, as preferrable to sometimes lax and chaotic measures overseas.

International flights are a fraction of pre-pandemic levels with arrivals undergoing strict weeks-long quarantine, and the mandatory track-and-trace apps mean close contacts are usually detected and quarantined quickly.

China is hewing strictly to the zero-tolerance approach as outbreaks continue to emerge in the run-up to next month's Beijing Winter Olympics.

China's official tally since the start of the pandemic -- just over 100,000 Covid cases -- is a fraction of the record one million cases logged by the US in a single day earlier this month.

Cases from China's chaotic initial outbreak in Wuhan in early 2020, however, are widely believed to have been under-reported.

China's official death toll has stayed under 5,000.