The pandemic killed an estimated 20 million people, according to WHO, while shredding economies and crippling health systems.

Understanding where the SARS-CoV-2 virus that causes Covid came from is seen as key to preventing future pandemics.

The first cases were detected in Wuhan in China in late 2019, with the WHO first characterising the outbreak as a pandemic in March 2020.

The WHO set up the Scientific Advisory Group for the Origins of Novel Pathogens (SAGO) to take the investigation into the pandemic's origins as far as they could.

Its report was being published on Friday.

WHO chief Tedros Adhanom Ghebreyesus said SAGO had advanced understanding of the origins of Covid-19 but much of the information needed to evaluate fully all the hypotheses had not been provided.

"Despite our repeated requests, China hasn't provided hundreds of viral sequences from individuals with Covid-19 early in the pandemic, more detailed information on animals sold at markets in Wuhan, and information on work done and biosafety conditions at laboratories in Wuhan," he told a press conference.

"WHO is also aware of intelligence reports performed by other governments around the world on the origins of Covid-19. We have also requested access to those reports.

"As things stand, all hypotheses must remain on the table, including zoonotic spillover and lab leak."

Tedros said the UN health agency continued to appeal to Beijing and other countries with information about the origins of Covid-19 to share the information openly, in the interests of protecting the world from future pandemics.