World Health Organization inspectors on Wednesday visited a laboratory in China's Wuhan city at the heart of a controversial theory that it could have been the source of the coronavirus.

The inspection of the Wuhan virology institute, which conducts research on the world's most dangerous diseases, will be one of the most-watched stops on the team's probe into the origins of the Covid-19 pandemic.

WHO inspectors are in China's Wuhan city to investigate the origins of Covid-19 WHO inspectors are in China's Wuhan city to investigate the origins of Covid-19 Photo: AFP / Hector RETAMAL

The sensitive mission, which China had delayed throughout the first year of the international health crisis, has a remit to explore how the virus jumped from animal to human.

But questions remain over what the experts can hope to find after so much time has passed.

Covid-19 was first detected in Wuhan in December 2019 Covid-19 was first detected in Wuhan in December 2019 Photo: AFP / Hector RETAMAL

The convoy of cars drove past security to enter a virology institute shrouded in mist Wednesday morning.

WHO team member Peter Daszak told reporters that the team was "looking forward to a very productive day and to asking all the questions that we know need to be asked".

Peter Daszak (R), Thea Fischer (L) and other members of the World Health Organization team investigating the origins of COVID-19, arrive at the Wuhan Institute of Virology Peter Daszak (R), Thea Fischer (L) and other members of the World Health Organization team investigating the origins of COVID-19, arrive at the Wuhan Institute of Virology Photo: AFP / Hector RETAMAL

He later tweeted that it had been an "extremely important meeting today with staff" and a "frank, open discussion."

Most scientists think Covid-19 -- which first emerged in Wuhan and has gone on to kill more than two million people worldwide -- originated in bats and could have been transmitted to people via another mammal.

Timeline on events, statements and discoveries relating to the origins of SARS-CoV-2, as an international team of WHO experts begin their much anticipated fieldwork in Wuhan Timeline on events, statements and discoveries relating to the origins of SARS-CoV-2, as an international team of WHO experts begin their much anticipated fieldwork in Wuhan Photo: AFP / John SAEKI

But there are no definitive answers so far.

The WHO experts stayed inside the institute for nearly four hours, before driving away without stopping to talk to media waiting outside.

Peter Daszak, a member of the World Health Organization team investigating the origins of the COVID-19, speaks to media upon arriving with other WHO members to the Wuhan Institute of Virology Peter Daszak, a member of the World Health Organization team investigating the origins of the COVID-19, speaks to media upon arriving with other WHO members to the Wuhan Institute of Virology Photo: AFP / Hector RETAMAL

Police in black uniforms and face masks lined the road to separate the crowds of reporters from the cars.

According to the state-run Global Times, the team also visited the P4 lab -- Asia's first maximum-security lab equipped to handle Class 4 pathogens (P4) such as Ebola.

World Health Organization inspectors arrive at the Wuhan virology institute World Health Organization inspectors arrive at the Wuhan virology institute Photo: AFPTV / Lillian DING

There was speculation early in the pandemic that the virus could have accidentally leaked from the biosafety lab in Wuhan, although there was no evidence to back up that theory.

Then-US president Donald Trump and his supporters seized on those rumours and amplified them with conspiracy theories that China deliberately leaked the virus.

The US-secretary of state Mike Pompeo insisted last year that there was "significant evidence" that the virus came from the lab, while releasing no proof and acknowledging that there was no certainty.

China has faced criticism at home and abroad for playing down the initial outbreak and concealing information when it first emerged in Wuhan in December 2019.

Beijing is now determined to put the focus on its recovery from the outbreak, and the WHO team toured a propaganda exhibition celebrating China's recovery from the pandemic in Wuhan on Saturday.

On Sunday the team went to the market where one of the first reported clusters of infections emerged over a year ago, which Daszak tweeted was a "critical" stop.

Other stops include the hospital which treated some of the first coronavirus cases.

Daszak said in his tweet that the team met Wednesday with Shi Zhengli, one of China's leading experts on bat coronaviruses and deputy director of the Wuhan lab.

Shi raised some eyebrows in a June 2020 interview with Scientific American magazine in which she said she was initially anxious over whether the virus had leaked from the facility.

She later said she would "bet her life that (the new coronavirus) had nothing to do with the lab", according to Chinese state media.