BEIJING - During a coronavirus outbreak in a Beijing wholesale market in June, Chinese health authorities pinned the blame on imported salmon, pulling hundreds of kilos of the fatty fish off supermarket shelves and menus.

This was initially met with scepticism by the international community, calling it China's attempt to explain away how infections were resurfacing despite declaring the Covid-19 situation mostly under control.

But in the five months since, there has been increasing evidence that the Sars-CoV-2 virus, which causes Covid-19, can survive on frozen and refrigerated food, given the right conditions.

There have been sporadic outbreaks across China, mostly linked to workers dealing with cold-chain imported food.

The country said last week that it would ban food imports from countries with coronavirus outbreaks in their production facilities, or whose products were found to contain traces of the virus.

Trade partners have bristled at the restrictions targeted at preventing imports of the virus, but China's severe measures should not be hastily written off: its travel bans and mandatory mask-wearing efforts earlier this year have proven prescient.

Food processing plants worldwide have seen coronavirus outbreaks among employees, who often work long hours in close proximity under hot, sweaty conditions.

Most of the sporadic outbreaks of Covid-19 in China were in port cities like Dalian, Qingdao and, most recently, Tianjin.

When two cold-chain storage workers were infected in Tianjin, a city just over an hour away from the capital Beijing, mass testing was rolled out for imported produce nationwide, while state media showed workers hosing down trucks with disinfectant and inspecting packets of frozen food.

In at least five cities, including Wuhan where Covid-19 first emerged at a wet market last December, the authorities have reported traces of the virus on frozen imported food, including beef from Brazil, pork from Argentina, and cuttlefish from India. These cities have so far not reported any cases of infection linked to the food imports.

Custom inspectors across China have so far tested more than 800,000 samples from refrigerated imports and suspended shipments from 99 overseas suppliers, senior customs official Bi Kexin told a press conference last week.

These suppliers were singled out because of coronavirus outbreaks at their processing plants, officials said.

The chief epidemiologist of the Chinese Centre for Disease Control and Prevention (CDC) said last week that there is growing evidence of the virus being imported on frozen seafood and meat products.

"(The Beijing outbreak) was the world's first discovery and confirmation that contaminated food can cause new Covid-19 outbreaks in other countries via cold-chain transportation," Dr Wu Zunyou said in an interview with the website of the Central Commission for Discipline Inspection.

Scientists from Singapore's National University Hospital have also found that frozen food could cause outbreaks in regions where the disease had been controlled. While the paper has not been peer reviewed, the team is now looking into whether the virus can survive on food packaging.

The finding, however, contradicts guidance from international health authorities: The World Health Organisation in April said it is "highly unlikely" that people can contract Covid-19 from food or food packaging.

Already, major food-producing countries are growing increasingly frustrated with China's scrutiny of imported products and are calling on it to stop aggressive testing for the coronavirus, which some say is tantamount to a trade restriction.

The United States, Brazil and New Zealand have been accused of having exported food that contained traces of coronavirus, but agricultural officials and diplomats say China has not been forthcoming with evidence.

On Tuesday, the US Department of Agriculture said China's restrictions were "not based on science" and threatened to disrupt trade. In response, China said the restrictions were "provisional based on scientific basis" and designed to "protect people's lives to the maximum extent".

Within China, there are also concerns over the impact of such bans on its food supply chain: The country has had to increase its produce imports this year because of swine flu and heavy flood that damaged swathes of farmland. Official data from September showed a 70 per cent increase in meat imports.

Yet ultimately, like many aspects of the pandemic - including the virus' origin, medical supplies and the race for a vaccine - there are signs of it turning into a geopolitical issue.

On Wednesday (Nov 18), the chief economist of the United Nations food and agriculture agency said there is no significant evidence of coronavirus being spread through food trade and such reports "need to be minimised".