China called the mounting international restrictions on travellers from its territory "unacceptable" on Tuesday after more than a dozen countries placed fresh Covid-19 curbs on visitors from the world's most-populous nation.
The United States, Canada, Japan and France are among the countries insisting all travellers from China provide negative Covid tests before arrival, as concerns grow over a surge in cases.
China's steep rise in infections comes after Beijing abruptly lifted years of hardline restrictions last month, with hospitals and crematoriums quickly overwhelmed.
But Beijing has pushed ahead with a long-awaited reopening, last week announcing an end to mandatory quarantines on arrival in a move that prompted Chinese people to plan trips abroad.
"Some countries have taken entry restrictions targeting China," foreign ministry spokesperson Mao Ning told a regular briefing.
"This lacks scientific basis and some practices are unacceptable," she added, warning China could "take countermeasures based on the principle of reciprocity".
Asked about China's reaction, France's Prime Minister Elisabeth Borne defended the new rules.
"I think we're performing our duty in asking for tests," Borne told franceinfo radio.
"We will continue to do it."
The rules imposed affect all travellers coming from China – not just Chinese nationals – while Beijing continues to restrict inbound visitors and not issue visas for tourists or international students.
Countries including the United States have also cited Beijing's lack of transparency around infection data and the risk of new variants as a reason to restrict travellers.
China has only recorded 22 Covid deaths since December and has dramatically narrowed the criteria for classifying such deaths – meaning that Beijing's own statistics about the unprecedented wave are now widely seen as not reflecting reality.
As health workers nationwide battle a surge in cases, a senior doctor at one of Shanghai's top hospitals said 70 percent of the megacity's population may now have been infected with Covid-19, state media reported on Tuesday.
Chen Erzhen, vice president at Ruijin Hospital and a member of Shanghai's Covid expert advisory panel, estimated that the majority of the city's 25 million people may have been infected.
"Now the spread of the epidemic in Shanghai is very wide, and it may have reached 70 percent of the population, which is 20 to 30 times more than [in April and May]," he told Dajiangdong Studio, owned by the Communist Party mouthpiece the People's Daily.
Shanghai suffered a gruelling two-month lockdown from April, during which more than 600,000 residents were infected and many were hauled to mass quarantine centres.
But now the Omicron variant is spreading rampantly across the city.
In other major cities, including Beijing, Tianjin, Chongqing and Guangzhou, Chinese health officials have suggested that the wave has already peaked.
In neighbouring Zhejiang province, disease control authorities said on Tuesday that there had been one million new Covid infections in recent days and that the province was entering a peak plateau.
Chen added that his Shanghai hospital was seeing 1,600 emergency admissions daily – double the number prior to restrictions being lifted – with 80 percent of them Covid patients.
"More than 100 ambulances arrive at the hospital every day," he was quoted as saying, adding that around half of emergency admissions were vulnerable people aged over 65.
At Tongren Hospital in downtown Shanghai, AFP reporters saw patients receiving emergency medical attention outside the entrance of the overcrowded emergency ward on Tuesday.
The corridors overflowed with dozens of elderly patients lying on beds crammed together, hooked up to IV drips.
At another hospital, AFP witnessed an exchange between a woman and an older man, both jostling for a drip.
"I was here first," she said. "I'm here to get a needle too."
Chinese officials are now readying for a virus wave to hit the country's underresourced rural interior, as millions of people prepare to travel to their hometowns for the week-long Lunar New Year public holiday beginning 21 January.
National Health Commission official Jiao Yahui admitted that dealing with the expected surge in rural areas would be an "enormous challenge".
"What we are most worried about is in the past three years nobody has returned home for Lunar New Year but they finally can this year," Jiao told state broadcaster CCTV on Monday.
"As a result, there may be a retaliatory surge of urban residents into the countryside to visit their relatives, so we are even more worried about the rural epidemic."