A leading scientist who was part of the World Health Organisation-led team to investigate the origins of coronavirus in Wuhan has cast further doubt the virus emerged in China.
Professor John Watson, who spent four weeks in Hubei province at the start of the year, said the virus's leap from animals to humans may have occurred outside the country's borders.
He says the pandemic most likely started with an infection in an "animal reservoir" which was then passed on to humans through an "intermediate host".
Asked if he was sure the virus emerged in China, Prof Watson, who previously served as England's deputy chief medical officer until 2017, said "no".
"There are all sorts of reasons ... that suggest that China is a very, very possible source for the outbreak," he told the BBC on Sunday.
"But by no means necessarily the place where the leap from animals to humans took place.
"And I think we need to ensure that we are looking beyond the borders of China, as well as within China."
“The supply chains to the Huanan seafood market were extensive, they were coming in from other countries, they were coming in from various parts of China, so to really trace that back it's going to take some work," he told the BBC last week.
The Huanan seafood market was shut down indefinitely at the beginning of 2020 as it was identified as the virus's first epicentre when multiple cases were linked back to the wet market.
In recent months, Chinese diplomats and state media have said they believe the market is not the origin but the victim of the disease, and have thrown support behind theories that the virus potentially originated in another country.
'Deep concerns' over China's transparency
Concerns have been raised about the WHO team's access to vital early data from the Chinese government.
US national security adviser Jake Sullivan said on Friday that Washington had "deep concerns about the way in which the early findings of the COVID-19 investigation were communicated and questions about the process used to reach them".
Meanwhile, the UK's Foreign Secretary Dominic Raab also shared concerns, saying scientists needed full co-operation to get the answers they need.
Prof Watson said the WHO team saw a "great deal" of information about the cases of the first 174 people who contracted coronavirus in China.
But he added that the team was only given access to a "certain amount" of the raw data.
"We didn't see all of that and we didn't see the original questionnaires that were used," he said.
"But apart from the fact that, of course, they would have been in Chinese, one has to think about what one would have seen if one had gone to any other country in the world."
He said the team's visit was not a "one-off" and that the WHO sees it as "the start of a process that's going to take really quite a while".
China has faced claims that the Wuhan Institute of Virology could be the suspected source of the COVID-19 virus.
But the WHO team concluded it was "extremely unlikely" to have entered the human population as a result of a lab-related incident.
Prof Watson said the possibility that it may have escaped from a laboratory had not been "ruled out".