The number of Covid-19 cases in China reached its highest level since March 2020 on Monday, as Beijing races to smother outbreaks just three weeks before hosting the Winter Olympics.

China, where the virus first emerged in late 2019, has stuck to a strict policy of targeting zero Covid cases even as the rest of the world has reopened.

But its approach has come under sustained pressure in recent weeks with multiple clusters across the country just as the Games are about to get under way in Beijing.

On Monday there were 223 more cases reported in China, including another 80 in the virus-hit port city of Tianjin, and nine more -- including cases of the highly transmissible Omicron variant -- in the southern manufacturing hub of Guangdong.

Athletes and officials have already started to land in the capital ahead of the Games, immediately entering a tightly controlled bubble separating them from the rest of the population.

But after a local Omicron case was detected in Beijing over the weekend, authorities have also tightened regulations for those arriving in the capital from elsewhere in China.

The city is now demanding a negative test before travel and a follow-up test after entering the city, with residents urged not to leave the city for the upcoming Lunar New Year holiday.

Some tourist sites in the capital have also been closed.

- Strain on economy -

The infected woman in Beijing had not travelled or had contact with infected people, authorities said, as they tested some 13,000 people living or working in the same area.

Health official Pang Xinghuo told reporters Monday the virus had been found on the surface of a letter the infected person had received from Canada, as well as inside the unopened letter.

Dozens of letters from the same batch were tested, and five showed positive traces of Covid-19, she said, including samples from inside unopened letters.

The strain was different from Omicron cases in China, and similar to strains identified from North America last month, she said.

"We come to the conclusion that the possibility of virus infection through inbound objects cannot be ruled out," she said.

Beijing's theory the virus did not originate in China but was imported in frozen food was judged "possible" but very unlikely in a report from World Health Organization-appointed international experts last year.

China has linked some virus clusters to products imported from overseas.

The Centers for Disease Control and Prevention in the US says on its website that it is "possible" for people to be infected through contact with contaminated surfaces or objects -- but the risk is low.

Within three days, there should be a 99 percent reduction of virus traces left on surfaces.

Analysts have warned that China's ongoing zero-Covid approach -- which includes swift and targeted lockdowns and travel restrictions  -- will increasingly weigh on the economy.

Another 68 cases were reported across central Henan province, where partial lockdowns and mass testing have been rolled out for millions of residents.

Zhuhai, the mainland city bordering the gambling hub of Macau, told residents to avoid leaving and started testing the whole city from Monday, after detecting a handful of Omicron cases.

Meanwhile in the historic northern city of Xi'an infections have slowed to single digits after nearly a month under lockdown.

Sixty more imported cases were also recorded, as China maintains strict controls over border entry including slashed flights and a "circuit breaker" policy where routes are halted if infections brought in are too high.