The WHO on Thursday urged China to share raw data from the earliest Covid-19 cases to revive the investigation into the pandemic's origins, and to release information to address the laboratory leak theory.
The World Health Organisation stressed it was "vitally important" to uncover the origins of the worst pandemic in a century.
Covid-19 has killed at least 4.3 million people and battered the global economy since the coronavirus that causes it was first detected in the Chinese city of Wuhan in December 2019.
In the face of opposition from Beijing, the UN health agency called for the provision of "all data and access required so that the next series of studies can be commenced as soon as possible".
After much delay, a WHO team went to Wuhan in January to produce a first-phase report, which was written in conjunction with Chinese experts.
Their March report drew no firm conclusions, instead ranking four hypotheses.
It said the virus jumping from bats to humans through an intermediate animal was the most probable scenario, while a leak from the Wuhan virology labs was "extremely unlikely".
But the investigation faced criticism for lacking transparency and access, and for not evaluating the lab-leak theory more deeply, with the US increasing the pressure ever since.
A WHO call last month for the investigation's second stage to include audits of the Wuhan labs infuriated Beijing.
Chinese vice health minister Zeng Yixin said the plan showed "disrespect for common sense and arrogance towards science".
The WHO, however, insisted the search was not "an exercise in attributing blame" or political point-scoring.
"The next series of studies would include a further examination of the raw data from the earliest cases and sera from potential early cases in 2019," the UN agency said.
"Access to data is critically important for evolving our understanding of science."
The WHO said it was working with several countries that reported detection of Sars-CoV-2 in samples from biological specimens stored in 2019.
It said that in Italy, it had organised an independent evaluation by international laboratories, which included the retesting of pre-pandemic blood samples.
"Sharing raw data and giving permission for the retesting of samples in labs outside of Italy reflects scientific solidarity at its best and is no different from what we encourage all countries, including China, to support so that we can advance the studies of the origins quickly and effectively," the WHO said.
After reading the phase-one report, WHO chief Dr Tedros Adhanom Ghebreyesus said the investigation into Wuhan's virology labs had not gone far enough.
Long derided as a right-wing conspiracy theory and vehemently rejected by Beijing, the hypothesis has been gaining momentum.
It was a favourite under former US president Donald Trump, but his successor Joe Biden is also keen to see this line of enquiry pursued.
"China and a number of other member states have written to WHO regarding the basis for further studies of the Sars-CoV-2 'lab hypothesis'," the WHO said.
"They have also suggested the origins study has been politicised, or that WHO has acted due to political pressure.
"In order to address the 'lab hypothesis', it is important to have access to all data and consider scientific best practice and look at the mechanisms WHO already has in place."
It said that analysing and improving lab safety and protocols "including in China, is important for our collective biosafety and security".
Meanwhile Danish scientist Peter Ben Embarek, who led the international mission to Wuhan, said a lab employee infected while taking samples in the field is one of the likely hypotheses as to how the virus passed from bats to humans.
Dr Ben Embarek told the Danish public channel TV2 that the suspect bats were not from the Wuhan region and the only people likely to have approached them were workers from the Wuhan labs.
He said that up until 48 hours before the end of the mission, the international and Chinese scientists could not agree on even mentioning the lab theory in the report.
