The World Health Organisation chief called Tuesday for investigators looking into the origins of the COVID-19 pandemic to delve deeper into a theory about a possible lab leak incident.

Tedros Adhanom Ghebreyesus welcomed a long-delayed report from an international mission tasked with probing how the virus that causes COVID-19 first jumped to humans, saying it "advances our understanding in important ways".

But in a briefing to member states, he stressed it also "raises further questions that will need to be addressed by further studies".

The report, compiled by WHO-appointed international experts and their Chinese counterparts, did not draw any firm conclusions, but did rank a range of hypotheses according to how likely they thought they were.

The report, seen by AFP ahead of its Tuesday publication, had judged a lab-leak hypothesis "extremely unlikely", saying the virus behind Covid-19 had most probably jumped from bats to humans via an intermediary animal.

But Tedros urged them to look deeper into the lab theory.

"Although the team has concluded that a laboratory leak is the least likely hypothesis, this requires further investigation, potentially with additional missions involving specialist experts, which I am ready to deploy," he said.

The WHO chief also voiced concern that the international expert team had "expressed the difficulties they encountered in accessing raw data" while in China.

"I expect future collaborative studies to include more timely and comprehensive data sharing," he said.