There is less evidence now to show that the novel coronavirus developed naturally — jumping from an animal to humans, strengthening the argument for Covid-19 originating in a lab, according to the former head of the US Food and Drug Administration, media reports said.

"The side of the ledger that suggests that this could have come out of a lab has continued to expand, and the side of the ledger that suggests this has come from of a zoonotic source, come out of nature, really hasn't budged, and if anything, you can argue that that side of the ledger has contracted," the New York Post quoted Scott Gottlieb as saying on on CBS' "Face the Nation."

"We've done an exhaustive search for this so-called intermediate host, the animal that could have been a host to this virus before it spread to humans, (and) we have not found such an animal.

"We've also fully disproven the market, the food market that was originally implicated in the outbreak as the source of the outbreak, so that side of the ledger probably has shrunken."

Gottlieb said that such accidents "happen all the time".

"Even here in the United States, we've had mishaps, and in China, the last six known outbreaks of SARS-1 have been out of labs, including the last known outbreak, which was a pretty extensive outbreak that China initially wouldn't disclose that it came out of a lab," he noted.

The Chinese government could have provided "exculpatory" evidence showing the virus that caused the pandemic didn't leak from a lab in Wuhan, but they are yet to do so, Gottlieb said.

"They could provide the blood samples from those who worked in the lab in Wuhan, they've refused to do that, they could provide the source strains, some of the original strains, they've refused to do that, they could provide access to some of the early samples that we can sequence, they can provide an inventory of what was in the lab (at) the Wuhan Institute of Virology, the lab that has been implicated in a potential lab leak, they have refused to do that," he said.

The US' top infectious disease expert Anthony Fauci has also said he is "not convinced" Covid-19 developed naturally.

US President Joe Biden, last week, ordered the intelligence community to re-double efforts to examine how the virus originated, including the lab accident theory.

The European Union, the UK, Australia and Japan also have joined the US in seeking a deeper probe into the origins of the pandemic.