An international expert team has set off for China to investigate the origins of the Covid-19 pandemic, but Beijing has yet to provide the necessary access, the WHO chief has said.

"Today, we learned that Chinese officials have not yet finalised the necessary permissions for the team's arrivals in China," Tedros Adhanom Ghebreyesus told reporters.

"I am very disappointed with this news, given that two members had already begun their journeys and others were not able to travel at the last minute," he said.

He stressed that he had been in contact with senior Chinese officials to make clear "that the mission is a priority for WHO and the international team".

"I have been assured that China is speeding up the internal procedure for the earliest possible deployment."

The World Health Organization has for months been working to send a team of international experts, including epidemiologists and animal health specialists, to China to help probe the animal origin of the novel coronavirus pandemic and how the virus first crossed over to humans.

But the mission is sensitive, and neither the WHO nor China had until now confirmed when specifically it was due to start, with the UN health agency only hinting it would take place during the first week of January.

WHO emergencies director Michael Ryan told the briefing that the problem was a lack of visa clearances.

He stressed that Dr Tedros had made clear in his discussions with Chinese officials "the absolute critical nature of this". 

"We trust and we hope that this is just a logistic and bureaucratic issue that can be resolved very quickly."

Covid-19 was first detected in the central city of Wuhan in late 2019, before seeping beyond China's borders to wreak havoc, costing over 1.8 million lives and eviscerating economies.

But its origins remain bitterly contested, lost in a fog of recriminations and conjecture from the international community, as well as obfuscation from Chinese authorities determined to keep control of its virus narrative.

The WHO team has promised to focus on the science, specifically how the coronavirus jumped from animals, believed to be bats, to humans.