The head of the World Health Organisation says "all hypotheses remain on the table" in the search for the origin of COVID-19.
Director-general Tedros Adhanom Ghebreyesus also said a WHO team looking for the source of the virus had encountered problems accessing raw data, and called for further studies because the assessment so far has not been extensive enough.
"We have not yet found the source of the virus, and we must continue to follow the science and leave no stone unturned as we do," he added.
Following the publication of an initial report from the WHO team, the UK joined other countries in expressing their "shared concerns" about delays and other problems encountered during the experts' study in China.
Beijing has previously been accused of a cover-up in its initial response to the COVID outbreak.
Meanwhile, delays to the arrival of the WHO team into China earlier this year were blamed by the country's officials on a "misunderstanding".
And one member of the WHO team in February accused China of refusing to hand over key data.
Earlier, Sky News revealed that experts believe the most likely origin of COVID-19 is transmission from a bat to humans via an unidentified intermediate animal species.
Scientists have concluded that a leak from a laboratory is the least likely hypothesis, after visiting several facilities.
Dr Tedros said the "role of animal markets is still unclear".
But he added that the WHO team had confirmed there was "widespread contamination with SARS-CoV-2 in the Huanan market in Wuhan".
The source of that contamination could not be determined, however.
He said further research may include a "full analysis of the trade in animals and products in markets across Wuhan, particularly those linked to early human cases".
Farmers, suppliers and their contacts need to be interviewed, he added, echoing a request from his team, which also addressed the possibility that the virus was introduced to humans through the food chain.
A team of scientists from the UK, Australia, China, Denmark, Germany, Japan, Kenya, Netherlands, Qatar, Russia, the US and Vietnam travelled to China in January, working with experts there for four weeks.
The results published on Tuesday are described as "initial studies".
Looking at available data, the report suggests there was unrecognised transmission in December 2019, and possibly earlier.
It adds that the first detected case began to show symptoms on 8 December 2019.
But to go further, the team needs "full access to data including biological samples from at least September 2019", Dr Tedros said.
He said the report was a "very important beginning" but not the end.
"Finding the origin of a virus takes time and we owe it to the world to find the source so we can collectively take steps to reduce the risk of this happening again," he added.
"No single research trip can provide all the answers.
"It is clear that we need more research across a range of areas, which will entail further field visits."
In a joint statement, Australia, Canada, the Czech Republic, Denmark, Estonia, Israel, Japan, Latvia, Lithuania, Norway, South Korea, Slovenia, the UK, and the US said they remained "steadfast" in their commitment to working with the WHO to uncover the origins of COVID-19.
They called on all WHO members to renew their commitment to "access, transparency, and timeliness" when investigating the origins of viruses.
The joint statement said: "The mission of the WHO is critical to advancing global health and health security, and we fully support its experts and staff and recognize their tireless work to bring an end to the COVID-19 pandemic, including understanding how the pandemic started and spread.
"With such an important mandate, it is equally essential that we voice our shared concerns that the international expert study on the source of the SARS-CoV-2 virus was significantly delayed and lacked access to complete, original data and samples.
"Scientific missions like these should be able to do their work under conditions that produce independent and objective recommendations and findings.
"We share these concerns not only for the benefit of learning all we can about the origins of this pandemic, but also to lay a pathway to a timely, transparent, evidence-based process for the next phase of this study as well as for the next health crises."