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US infectious disease expert Dr Anthony Fauci faced intense questioning from Republican lawmakers in his first public appearance before Congress since leaving his White House advisory role in 2022.

For more than a year, he has faced scrutiny from Republicans in the House Select Subcommittee on the Coronavirus Pandemic for his role in the pandemic response.

In his Monday testimony, Dr Fauci forcefully rejected allegations that he tried to hide information about the virus's origins, calling them "simply preposterous".

The origins of the pandemic have been a matter of strong debate in the US.

During the pandemic, Dr Fauci, who is now retired, became the face of the US Covid-19 response and has been the subject of both praise and fierce criticism.

Before lawmakers on Monday, he disputed claims there was any attempt to cover up evidence suggesting the virus may have leaked from a laboratory in Wuhan, China.

He read from an email he sent in February 2020 where he spoke about encouraging a colleague worried about a lab leak to examine the evidence "as soon as possible" and, if the concerns were valid, report the findings to the "appropriate authorities".

He told the subcommittee: "It is inconceivable that anyone who reads this email could conclude that I was trying to cover up the possibility of a laboratory leak."

During the sometimes heated hearing, Republicans also pressed Dr Fauci on allegations that he used his personal email to conduct official business, which he denied.

Questions about his emails come after messages from a former adviser, in which the adviser discussed with colleagues using personal email to avoid disclosure under public records laws, came to light.

Dr Fauci said he had not been aware that the adviser used an unofficial email.

The adviser, Dr David Morens, faced criticism from lawmakers from both parties in testimony last month before the panel.

Dr Fauci also faced questions about National Institute of Health (NIH) funding for virology research at the Wuhan lab.

An US-based organisation - EcoHealth Alliance - was awarded a grant in 2014 from the NIH to look into possible coronaviruses from bats. Some funds were given to the Wuhan Institute of Virology in an agreement that was terminated in April 2020.

"I have always said, and will say now, I keep an open mind as to what the origin [of Covid-19] is," Dr Fauci said on Monday.

"But the one thing I know for sure is that the viruses that were funded by the NIH, phylogenetically, could not be the precursor of SARS-CoV-2."

Dr Fauci received support from Democratic lawmakers on the subcommittee, who accused some Republicans of making irresponsible claims.

"Over the past four years you have been personally targeted by extreme narratives of the origins of the Covid-19 pandemic and the US governments response to it," Democrat Representative Raul Ruiz said.

The infectious disease expert also spoke about threats he and his family have faced since the pandemic.

"There have been credible death threats, leading to the arrests of two individuals. And credible death threats means someone who was clearly on their way to kill me," Dr Fauci said.

"Every time someone gets up and says I am responsible for the death of people throughout the world, the death threats go up."

In a declassified report released last year, US intelligence agencies found no direct evidence that Covid-19 broke out from a Chinese laboratory.

The Office of the Director of National Intelligence said both a natural and laboratory origin remain plausible scenarios.

More than 1.2 million American deaths have been linked to Covid.