The statement came days after a World Health Organization team of inquiry returned from Wuhan, China, the epicenter of the pandemic with no clear finding on the origin of the virus.

Members had to walk a diplomatic tightrope during their stay, with the US urging a "robust" probe and China warning against politicizing the issue.

"We have deep concerns about the way in which the early findings of the COVID-19 investigation were communicated and questions about the process used to reach them," Sullivan said.

"It is imperative that this report be independent, with expert findings free from intervention or alteration by the Chinese government," he continued.

And he called on China to "make available its data from the earliest days of the outbreak."

Despite failing to find the virus origins a year after the pandemic began, the team of foreign experts did agree it likely jumped from bats to an unknown animal species before transmitting to humans.

The theory of a lab experiment gone wrong -- a theory voiced by then US president Donald Trump and his secretary of state Mike Pompeo -- was "extremely unlikely," the team said, while introducing new avenues of inquiry.

Beijing has repeatedly floated the theory that the virus was instead brought to China through packaging on products such as frozen seafood -- a theory the WHO team did not rule out.

Sullivan expressed "deep respect" for the WHO, which the US is rejoining after the Trump administration quit it to protest what it said was Beijing's ineffectual virus response.

But, Sullivan added, "re-engaging the WHO also means holding it to the highest standards. And at this critical moment, protecting the WHO's credibility is a paramount priority."