Data was withheld from World Health Organisation investigators who travelled to China to research the origins of the coronavirus epidemic, WHO director-general Tedros Adhanom Ghebreyesus said on Tuesday.

He made the comments to the agency’s member states as a WHO-led team that spent four weeks in and around Wuhan, China, in January and February released its final report to the public.

China refused to give raw data on early Covid-19 cases to the WHO-led team, one of the team’s investigators has already said, potentially complicating efforts to understand how the global pandemic began.

“In my discussions with the team, they expressed the difficulties they encountered in accessing raw data,” Tedros said. “I expect future collaborative studies to include more timely and comprehensive data sharing.”

The conclusions that the virus origins remains incomplete likely means that tensions over how the pandemic started — and whether China has helped or hinder efforts to find out, as the US has alleged — will continue.

Although the team concluded a leak from a Wuhan-area laboratory was the least likely hypothesis, Tedros said, the matter requires further investigation.