Donald Trump has said his government is trying to determine whether the coronavirus emanated from a lab in Wuhan.
Secretary of State Mike Pompeo, meanwhile, said Beijing "needs to come clean" on what they know - with the source of the killer virus still a mystery.
General Mark Milley, chairman of the Joint Chiefs of Staff, said on Tuesday that US intelligence indicates that the Covid-19 likely occurred naturally, and not in a lab.
But he said there is no certainty either way.
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Fox News reported on Wednesday that the virus originated in a Wuhan laboratory - but not as a bioweapon.
Instead, the American news network said it was part of China's effort to demonstrate that its efforts to identify and combat viruses are equal to or greater than the capabilities of the US.
This report and others have suggested the Wuhan lab where virology experiments take place is the origin.
They allege lax safety standards there led to someone getting infected and appearing at a nearby "wet" market, where the virus began to spread.
At a White House news conference on Wednesday, President Trump was asked about the reports of the virus escaping from the Wuhan lab, and he said he was aware of them.
"We are doing a very thorough examination of this horrible situation that happened," he said.
Asked if he had raised the subject in his conversations with Chinese President Xi Jinping, Trump said: "I don't want to discuss what I talked to him about the laboratory.
"I just don't want to discuss, it's inappropriate right now."
Trump has sought to stress strong US ties with China during the pandemic as the United States has relied on China for personal protection equipment, desperately needed by American medical workers.
As far back as February, the Chinese state-backed Wuhan Institute of Virology dismissed rumors the virus may have been artificially synthesized at one of its labs or escaped from such a facility.
Pompeo, in a Fox News Channel interview after Trump's news conference, said "we know this virus originated in Wuhan, China,".
He added that the Institute of Virology is only a handful of miles away from the wet market.
Pompeo said "we really need the Chinese government to open up" and help explain "exactly how this virus spread".
"The Chinese government needs to come clean," he added.
The broad scientific consensus holds that SARS-CoV-2, the virus' official name, originated in bats.
Trump and other officials have expressed deep skepticism of China's officially declared death toll from the virus of around 3,000 people, when the United States has a death toll of more than 20,000 and rising.
He returned to the subject on Wednesday, saying the United States has more cases "because we do more reporting".
"Do you really believe those numbers in this vast country called China, and that they have a certain number of cases and a certain number of deaths - does anybody really believe that?'' he said.
