President Donald Trump and some officials are flirting with a theory that coronavirus was set loose on the world by a Chinese lab that let it escape.

Without presenting evidence, they are trying to blame China for sickness and death from COVID-19 in the United States.

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"More and more, we're hearing the story," Trump says.

Secretary of State Mike Pompeo adds, "The mere fact that we don't know the answer - that China hasn't shared the answers - I think is very, very telling."

A scientific consensus is still evolving. But experts overwhelmingly say analysis of the coronavirus's genome rules out the possibility it was engineered by humans, as some conspiracy theories suggest.

Nor is it likely the virus emerged from a negligent laboratory in China, they say.

"I would put it on a list of 1,000 different scenarios," said Nathan Grubaugh of Yale University, who studies the epidemiology of microbial disease.

Bats to people

Scientists say the virus arose naturally in bats. The leading theory is infection among humans began at an animal market in Wuhan, China, probably from an animal that got the virus from a bat.

Even so, Pompeo and others are pointing fingers at an institute run by the Chinese Academy of Sciences that has done groundbreaking research tracing the likely origins of the SARS virus, finding new bat viruses and discovering how they could jump to people.

"We know that there is the Wuhan Institute of Virology just a handful of miles away from where the wet market was," Pompeo said Wednesday on Fox News.

US officials say the American Embassy in Beijing did flag concerns about potential safety issues at the lab in Wuhan in 2018, but stressed there is no evidence the virus originated there nearly two years later.

The episode shows that the two world powers are not above floating shaky theories, likely to divert attention from problems in their pandemic response. China previously spread the notion the virus had been started by the US.

Both countries had wasted crucial time responding to the outbreak.

More than 3,000 people were infected before China's government told the public what it had concluded six days earlier - that a pandemic was probably coming.

'Be transparent!'

Beijing muffled early warnings, to the extent that the Chinese people were assured the risk of sustained human-to-human transmission was low even as infected people entered hospitals across the country and the first case outside China was found in Thailand.

Chinese leaders tried blaming the US. "It might be US army who brought the epidemic to Wuhan," China's foreign ministry spokesman, Zhao Lijian, tweeted on March 12. "Be transparent! Make public your data! US owe us an explanation!"

China subsequently stopped making that accusation internationally.

The US, also late to take the threat seriously, has lagged behind a number of other countries in the thick of the pandemic when it comes to its response.

Trump failed to live up to his early promises to have ample testing, a key factor in containing the disease. The US still struggles to supply hospitals, front-line workers, and patients with necessities in a climate of confusion.

More than 640,000 people in the US have been infected with COVID-19 - not counting large numbers whose illnesses are not being registered - and more than 31,000 have died.

Against that backdrop, the pressure for scapegoats is strong.

After weeks of elaborate praise of Chinese President Xi Jinping's performance in the pandemic, Trump has turned to blaming China and halting US contributions to the World Health Organization, accusing it of parroting misinformation from Beijing.

Nefarious genetic engineering?

In the US, claims that the virus was created in or released from a Chinese lab emerged just weeks after the outbreak began and quickly spread from fringe internet sites to the wider public, abetted by conspiracy theorists of every stripe.

The reality is more mundane, said Dr Gregory Poland, head of vaccine research at the Mayo Clinic in the US state of Minnesota.

"This virus is a typical bat coronavirus that has developed the capacity to infect other mammals," he said. "What's becoming evident is that the natural origin of this fits with the transmission dynamics and biology of it all."

Whatever they think about the idea of a laboratory leak, Trump officials have not taken up the theory that China might have created or released the virus through nefarious genetic engineering or ill intent.

"I don't have much faith that they're even being truthful with us now," Defense Secretary Mark Esper said Thursday on NBC's Today show. Yet, "a majority of the views right now is that it is natural, it was organic".

"Once we get beyond the pandemic we'll have a chance to look back and really find out what happened," said Esper.