The US envoy to China has called on the country to be "more honest" about the origins of the Covid-19 virus.
His comments come a day after US media reported that a federal agency had found the pandemic probably started from a laboratory leak in Wuhan.
The Department of Energy reportedly concluded with "low confidence" that the virus was accidentally leaked.
China's foreign ministry hit back, saying covid's origin "was about science and should not be politicised".
Washington-Beijing ties have been under strain since the US shot down an alleged Chinese spy balloon this month.
Ambassador Nicholas Burns told a US Chamber of Commerce event on Monday that China needs to "be more honest about what happened three years ago in Wuhan with the origin of the Covid-19 crisis".
The energy department had formerly said it was undecided on how the virus began.
Other US agencies have drawn differing conclusions, with varying degrees of confidence in their findings. The FBI in 2021 concluded with "moderate confidence" that the virus leaked from a lab.
Other studies suggest it made the leap from animals to humans at Wuhan's Huanan seafood and wildlife market.
An unclassified report released by the US top spy official in October 2021 said that four US intelligence agencies had assessed with "low confidence" that it had originated with an infected animal or a related virus.
White House national security spokesman John Kirby said on Monday there was still no firm finding either way.
"There has not been a definitive conclusion and consensus in the US government on the origins of the Covid-19 pandemic," he told reporters when asked about the energy department's reported determination.
A spokeswoman for China's foreign ministry on Monday again rejected the lab leak theory.
Mao Ning called on US investigators to "stop smearing China and stop politicising origins-tracing".
After it was reported that the US energy department had settled on the lab leak theory, Republican Senator Tom Cotton tweeted "being proven right doesn't matter".
"What matters is holding the Chinese Communist Party accountable so this doesn't happen again."
Massachusetts Democratic Congressman Seth Moulton told CNN he was "not entirely surprised" by the energy department's reported conclusion.
"The Chinese have mishandled Covid at every step of the way, [and] are trying to sweep it under the rug," he said.
