Beijing has retaliated against South Korea's COVID-19 curbs on travellers from China, while state media further downplays the severity of the outbreak in the last major economy to reopen its borders after three years of isolation.

China ditched mandatory quarantines for arrivals and allowed travel to resume across its border with Hong Kong on Sunday, removing the last major restrictions under the "zero-COVID" regime which it abruptly began dismantling in early December after historic protests against the curbs.

But the virus is spreading unchecked among its 1.4 billion people and worries over the scale and impact of its outbreak have prompted South Korea, the United States and other countries including Australia to require negative COVID-19 tests from travellers from China.

Although China imposes similar testing requirements for all arrivals, foreign ministry spokesperson Wang Wenbin told reporters on Tuesday the entry curbs for Chinese travellers were "discriminatory".

"We will take reciprocal measures," Wang said, without elaborating.

The Chinese embassy in South Korea has suspended issuing short-term visas for South Korean visitors, it said on Tuesday - the first retaliatory move against nations imposing COVID-19 curbs on travellers from China.

The embassy will adjust the policy subject to South Korea lifting "discriminatory entry restrictions" against China, it said on its official WeChat account.

Kyodo news agency, quoting multiple travel industry sources, said China has told travel agencies it has stopped issuing new visas in Japan.

China has been reporting five or fewer deaths a day since the policy U-turn, figures that have been disputed by the World Health Organisation and are inconsistent with funeral homes reporting surging demand.

Some governments have raised concerns about Beijing's data transparency as international experts predict at least one million deaths in China this year.

Washington has also raised concerns about future potential mutations of the virus.

China dismisses criticism over its data as politically-motivated attempts to smear its "success" in handling the pandemic and said any future mutations are likely to be more infectious but less harmful.

State media also downplayed the severity of the outbreak.

An article in Health Times, a publication managed by People's Daily, the ruling Communist Party's official newspaper, quoted several officials as saying infections have been declining in the capital Beijing and several Chinese provinces.

Kan Quan, director of the Office of the Henan Provincial Epidemic Prevention and Control, said almost 90 per cent of people in the central province of 100 million people had been infected as of January 6.

Beijing acting mayor Yin Yong said the capital was also past its peak.

Li Pan, from the Municipal Health Commission in the city of Chongqing, said the peak there was reached on December 20.

In the eastern province of Jiangsu, the peak was reached on December 22, while in neighbouring Zheijiang province "the first wave of infections has passed smoothly", officials said.

Although daily flights in and out of China are still at one-tenth of pre-COVID levels, businesses across Asia celebrated the prospect of more Chinese tourists.

Chinese shoppers spent $US250 billion ($A361 billion) a year overseas before COVID.

The border rules were not the only COVID conflict brewing in China, with state media lashing out at Pfizer Inc over the price of its virus treatment Paxlovid.

Pfizer chief executive Albert Bourla said on Monday the company was in discussions with Chinese authorities about a price for Paxlovid, but not over licensing a generic version in China.