The head of the World Health Organization (WHO) says further investigation is needed to conclusively rule out a theory that Covid-19 emerged from a laboratory in China.
UK and US criticise WHO's Covid report and accuse China of withholding data.
Tedros Adhanom Ghebreyesus said on Tuesday that although a laboratory leak is the least likely cause, more extensive research is needed.
The virus was first detected in Wuhan, in China's Hubei province in late 2019.
The Chinese government has dismissed the allegations of a virus leak.
Since the novel coronavirus was first identified, more than 2.7 million people are known to have died from it, with more than 127 million cases worldwide.
An international team of experts travelled to Wuhan in January to probe the origins of the virus. Their research relied on samples and evidence provided by Chinese officials.
Dr Tedros said the team had difficulty accessing raw data and called for "more timely and comprehensive data sharing" in the future.
The US and the UK have sharply criticised a World Health Organization report into the beginnings of the coronavirus pandemic in Wuhan, implicitly accusing China of “withholding access to complete, original data and samples”.
The statement, also signed by 12 other countries including Australia and Canada, came hard on the heels of an admission on Tuesday by the head of the WHO, Tedros Adhanom Ghebreyesus, that the investigation was “not extensive enough” and experts had struggled to access raw information during their four-week visit to Wuhan in January.
Tedros also said there should be continued examination of the theory that the virus had escaped from a Wuhan institute of virology laboratory, even though the report deemed it “extremely unlikely” as a source of the pandemic – a theory promoted by some in the Trump administration.