The United States, which has accused China of having hidden the extent of its coronavirus outbreak, called on Tuesday for a "transparent and inclusive" WHO-led international investigation into the origin of the pandemic, criticising its current terms.
The Trump administration has accused the World Health Organization of being "China-centric" and of being its puppet, which WHO director-general Tedros Adhanom Ghebreyesus has denied.
The virus, known as SARS-CoV-2, is believed to have emerged in the Chinese central city of Wuhan late last year, possibly from bats at a market with live animals.
Chinese scientists are carrying out research into its origins and how it jumped the species barrier. A WHO-led international team formed in September is to develop plans for longer-term studies building on China's findings, according to the WHO's published terms of reference.
Garrett Grigsby, head of the global affairs office at the US Department of Health and Human Services, told the WHO's ministerial assembly that member states had been informed of the investigation's terms of reference only a few days ago.
The terms were "not negotiated in a transparent way with all WHO member states" and "the investigation itself appears to be inconsistent" with its mandate, he said, without elaborating.
"Understanding the origins of COVID-19 through a transparent and inclusive investigation is what must be done to meet the mandate," Grigsby said.
Britain called for prioritising the probe, adding: "We expect the investigation and its outcomes to be grounded in robust science."
Sun Yang, of China's National Health Commission, did not mention the investigation in his speech on Tuesday, but said that China supports "WHO's continued leadership role".
German Health Minister Jens Spahn, speaking for the European Union on Monday, called for "full transparency and cooperation" during all phases of the investigation.
WHO's top emergency expert Mike Ryan said on October 30 that the WHO-led team and its Chinese counterparts had held a first virtual meeting regarding joint investigations and would deploy on the ground in time.
A separate independent panel said on Tuesday it was working to establish "an accurate and authoritative chronology" behind the first outbreaks and responses. Former New Zealand Prime Minister Helen Clark and former Liberian President Ellen Johnson Sirleaf were named in July to co-lead the WHO panel.
Trump announced a temporary halt to US funding to the WHO in April, prompting condemnation from many world leaders. The United Nations said in July it had received formal notification of the US decision to leave the body next year.
