The United Nations Friday called on all member states, including China, to fully cooperate with the World Health Organization (WHO) amid Beijing's rejection of a further probe into the origins of the Covid-19.

"We implore all member states, including China, to cooperate fully with the World Health Organization, and if the World Health Organization believes it requires further information, we hope that they will all cooperate," UN deputy spokesperson Farhan Haq, was quoted as saying by news agency Sputnik, in a press briefing Friday.

The WHO earlier this month proposed a second phase of studies into the origins of the coronavirus in China, including audits of laboratories and markets in the city of Wuhan, calling for transparency from authorities.

Beijing meanwhile on Thursday rejected WHO's proposal. China's National Health Commission rejected the WHO plan for a second phase of the investigation, claiming its criteria were "insulting" and heavily suggested the laboratory leak possibility.

On Saturday, WHO chief Tedros Adhanom Ghebreyesus said that the second study will include an audit of labs and research facilities in the area where the first cases of Covid-19 were registered in December 2019. The new study is part of a five-stage plan which the WHO put together to identify where the origins of the coronavirus.

In recent months, the lab-leak theory has gained traction pushing US President Joe Biden to give the American intelligence team a 90-day deadline to find answers into the virus origin.

A WHO-led team of scientists that travelled to China in early 2021 to investigate the origins of the virus struggled to get a clear picture of what research China was conducting beforehand, faced constraints during its visit, and had little power to conduct thorough and impartial research.