The conclusion - noted in a classified report by Director of National Intelligence Avril Haines's office, The Wall Street Journal reported - marks a shift by the Energy Department, which had previously said it was undecided on how the virus emerged.
People who read the classified report were quoted in the Journal and The New York Times as saying the department made its judgement with "low confidence," highlighting how different agencies remain divided over the origins of Covid-19 and the pandemic that swept the globe in early 2020.
The conclusion, reportedly the result of new intelligence, is nevertheless significant because the department works with a network of national laboratories, including some that do advanced biological research.
The department now joins the Federal Bureau of Investigation in believing that the pandemic, which has left nearly seven million people dead, was the result of a mishap in a Chinese laboratory.
Beijing again denied the claim, with a foreign ministry spokesperson telling people to "stop stirring up claims about laboratory leaks, stop smearing China, and stop politicizing the origin-tracing issue."
"A laboratory leak wasn't found to be possible by authoritative scientific conclusions drawn by joint experts from China and the (World Health Organization)," Mao Ning said at a press conference.
The WHO said it had not received any information about the Energy Department's findings and was continuing to examine "all available scientific evidence," spokesperson Tarik Jasarevic told AFP on Monday.
"We call on China and the scientific community to undertake necessary studies" for determining the virus' origin, he added. "Until we have more evidence, all hypothesis are still on the table."
The White House maintained Sunday that American intelligence remained divided on the issue.
Four US intelligence agencies believe Covid occurred through natural transmission, while two others remain undecided, the Journal reported.
White House National Security Advisor Jake Sullivan emphasized a "variety of views" on the matter remain.
"Right now, there is not a definitive answer that has emerged from the intelligence community on this question," he told CNN Sunday.
In mid-February, the WHO pledged to do everything possible "until we get the answer" on Covid's origins, denying a report suggesting the agency had abandoned its investigation.
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.
China Rejects US Covid Blame
FILE: Security personnel keep watch outside Wuhan Institute of Virology during the visit by the World Health Organization (WHO) team tasked with investigating the origins of the coronavirus disease (COVID-19), in Wuhan, Hubei province, China. Taken Feb.3, 202
China on Monday rejected claims that the Covid-19 pandemic arose from a leak at one of its laboratories, following media reports that the US Department of Energy has determined that was the most likely cause of the outbreak.
BEIJING —