Beijing accused Washington on Wednesday of harming its own credibility after a top US intelligence official said his agency believed the pandemic "most likely" caused by a laboratory incident in Wuhan, China.
FBI Director Christopher Wray told Fox News on Tuesday that the Bureau has now assessed the source of Covid-19 was "most likely a potential lab incident in Wuhan."
Chinese officials have angrily denied the claim, calling it a smear campaign against Beijing.
"The United States once again stirs up the laboratory leak theory, which will not discredit China, which will further lower its own credibility," foreign ministry spokesperson Mao Ning told a regular briefing on Wednesday.
Wray's comments come after a report earlier this week said the US Department of Energy had determined that a leak from a Chinese lab was the most likely cause of the Covid-19 outbreak.
The department's findings are significant because it works with a network of national laboratories, including some that do advanced biological research.
But other agencies within the US intelligence community believe the virus emerged naturally in the world.
In Tuesday's interview, Wray also accused the Chinese government of trying to stall US efforts to investigate the causes of the pandemic.
"The Chinese government... has been doing its best to try to thwart and obfuscate the work here, the work that we're doing, the work that our US government and close foreign partners are doing," Wray said.
"And that's unfortunate for everybody."
At Wednesday's press briefing, Mao reiterated a longstanding and unsubstantiated Chinese claim that the virus could have escaped from the US military research lab at Fort Detrick, Maryland.
"The US should respect science and facts, cooperate with the World Health Organization as soon as possible, invite international experts to conduct traceability research in its country, and share research results with the international community," she said.
The scientific community sees it as crucial to determine the origins of the pandemic in order to better fight or even prevent the next one.