GENEVA, Switzerland -- Just days after Beijing rejected calls for a renewed probe into the origins of Covid-19, the World Health Organization said Wednesday it remained confident China would cooperate in the investigation.

Pressure has again been mounting on Beijing to consider a fresh probe into the origins of a pandemic which has killed more than four million people and paralysed economies worldwide since it first emerged in the central Chinese city of Wuhan in late 2019.

"I'm confident... that our colleagues in China are very much willing to cooperate on the scientific studies that are needed to further explore the origins," WHO emergencies director Mike Ryan told reporters.

His comment came after Beijing last Friday rejected as political a call from the UN health agency for raw data from the earliest Covid-19 cases to help revive the origins probe.

The WHO, and Ryan in particular, has repeatedly appealed for a depoliticisation of the origins investigation -- seen as vital to helping prevent future pandemics.

"I think what's happened in all of this is that the politics have really contaminated the environment and changed the atmosphere," he said Wednesday.

"We're working very hard behind the scenes to increase the level of confidence and to get people to recommit to the scientific process," he said.

"I believe we're making headway in that," he said, acknowledging though that the task was "not that easy given some of the rhetoric that we've all experienced over the last number of weeks and months." The rhetoric and finger-pointing contributed to delaying the first phase of the investigation, with the WHO only able to send a team of international experts to Wuhan in January 2021, more than a year after the pandemic erupted.

The team's report, written in conjunction with their Chinese counterparts, drew no firm conclusions, instead ranking four hypotheses.

It said the virus jumping from bats to humans via an intermediate animal was the most probable scenario, while a leak from the Wuhan virology labs was "extremely unlikely".

However, the investigation faced criticism for lacking transparency and access, and for not evaluating the lab-leak theory more thoroughly -- with the United States upping the pressure ever since.

In the face of China's reluctance to open up to outside investigators, experts are increasingly open to considering the theory that the virus might have leaked out of a lab, once dismissed as a conspiracy propagated by the US far-right.

Even WHO chief Tedros Adhanom Ghebreyesus has said that the initial probe into Wuhan's virology labs had not gone far enough, while President Joe Biden in May ordered a separate investigation into the virus origins from the US intelligence community.

A WHO call last month for the investigation's second stage to include audits of the Wuhan labs infuriated Beijing, with vice health minister Zeng Yixin saying the plan showed "disrespect for common sense and arrogance towards science".