The scientists who studied the origins of COVID-19 for the World Health Organization (WHO) warned on Wednesday that the window of opportunity for conducting further studies of how the virus emerged was "closing fast".
The team of international scientists travelled to China for 28 days in January to speak with professionals and visit institutions to understand the first outbreak of the coronavirus in Wuhan, China in December 2019.
In a report published in March, the team said that the virus likely jumped to humans from animals and that the scenario that it leaked from a lab was "extremely unlikely".
The scientists wrote in Nature on Wednesday that the March report was "meant to be the first step in a process that has stalled."
"The Chinese team was and still is reluctant to share raw data (for instance, on the 174 cases identified in December 2019), citing concerns over patient confidentiality," the scientists said.
They also pointed out that in their view the hypothesis that the virus jumped from animals to humans has more evidence to support it than the lab leak theory.
They warned that time was running out to do antibody surveys, for instance, and that many Chinese wildlife farms have also been closed in response to the pandemic, they said, with animals culled.
The commentary was published as US media reported that an intelligence report ordered by President Joe Biden on the origins of the virus was inconclusive.
China has responded by accusing the US of politicising the issue, the AP reported.