The World Health Organization said Friday that the second stage of an investigation into the origins of coronavirus should include further studies in China and lab "audits".

In a closed-door briefing to member states, WHO chief Tedros Adhanom Ghebreyesus proposed five priorities for the next phase of the investigation.

They included "audits of relevant laboratories and research institutions operating in the area of the initial human cases identified in December 2019", according to a copy of his opening statement provided by WHO.

He also suggested investigators should focus on "studies prioritising geographic areas with the earliest indication of circulation of SARS CoV-2".

And he called for more studies of animal markets in and around the Chinese city of Wuhan, where Covid-19 was first detected.

The UN health agency has been under intensifying pressure for a new, more in-depth investigation of Covid-19's origins.

The WHO only managed to send a team of independent, international experts to Wuhan in January, more than a year after Covid-19 first surfaced there in late 2019, to help their Chinese counterparts probe the pandemic origins.

The long-delayed report after the first phase of the investigation was published in late March, but drew no firm conclusions about how the virus that causes Covid first jumped to humans.

Instead they ranked several hypotheses according to how likely they believed they were, finding that it was most likely the virus jumped from bats to humans via an intermediate animal, while a theory involving the virus leaking from a laboratory was deemed "extremely unlikely".

The investigation and report have faced criticism for lacking transparency and access, and for not evaluating the lab-leak theory more deeply.

Tedros, who has always maintained that all theories remained on the table, told journalists Thursday that the push to rule out the possible link to a lab leak had been "premature".

Foreign ministry spokesman Zhao Lijian rejected that, standing by the first mission's conclusion that Covid-19 having escaped from a lab was "extremely unlikely", warning that "this issue should not be politicised."