China refused to hand over key data to the World Health Organization (WHO) team investigating the origins of Covid-19, one of its members has said.
Microbiologist Dominic Dwyer told Reuters, the Wall St Journal and the New York Times the team requested raw patient data from early cases, what he called "standard practice".
He said they only received a summary.
China has not responded to the allegation but has previously insisted it was transparent with the WHO.
The US has urged China to make available data from the earliest stages of the outbreak, saying it has "deep concerns" about the WHO report.
Last week, the WHO team concluded it was "extremely unlikely" that the coronavirus leaked from a lab in the city of Wuhan, dismissing a controversial theory that emerged last year.
Wuhan was the first place in the world where the virus was detected, in late 2019. Since then, more than 106 million cases and 2.3 million deaths have been reported worldwide.
What did the WHO team want to see?
The investigators had asked for raw data on the 174 identified cases of Covid-19 from Wuhan in December 2019, Professor Dwyer told Reuters.
Only half of the early cases had been exposed to the seafood market where the virus was initially detected.
"That's why we've persisted to ask for that," Prof Dwyer said. "Why that doesn't happen, I couldn't comment. Whether it's political or time or it's difficult... But whether there are any other reasons why the data isn't available, I don't know. One would only speculate."
Thea Kolsen Fischer, a Danish immunologist who was also part of the WHO team, told the New York Times that she saw the investigation as "highly geopolitical".
"Everybody knows how much pressure there is on China to be open to an investigation and also how much blame there might be associated with this," she said.
Prof Dwyer said the restrictions on data would be mentioned in the WHO team's final report, which could be released as early as next week.
The team arrived in early January and spent four weeks in China - the first two in hotel quarantine.
What has been the response from the Chinese?
Beijing has insisted it was transparent with the WHO investigators, whose visit began only after months of negotiations. The experts were closely monitored by the Chinese authorities.
The US accused China of hiding the extent of the initial outbreak and criticised the terms of the visit, which restricted the freedom of the WHO team to travel and interview witnesses, including community members, on health grounds.
The investigators told the New York Times that disagreements, including over access to patient records, were so tense that they sometimes erupted into shouting matches.
Last month, an interim report from the WHO criticised China's initial response, saying that "public health measures could have been applied more forcefully".
The WHO team has also called for further investigation into the possibility of "cold chain" transmission, referring to the theory the virus could have spread through the transport and trade of frozen food.
Dr Peter Daszak, a member of the WHO team, said the focus on where the origins that led to Covid-19 might be, could be shifted to South East Asia.