The coronavirus pandemic probably arose from a Chinese laboratory leak, the US Department of Energy announced on Sunday, although sources who have seen its classified report said the agency made its judgment with "low confidence".
The White House maintained that US intelligence remained divided on the issue.
The Wall Street Journal reported that the conclusion marks a shift by the energy department, which had previously said it was undecided how the virus emerged.
The conclusion, reportedly the result of new intelligence, is nevertheless significant because the department oversees a network of national laboratories, including some that conduct advanced biological research.
The FBI believes that the pandemic, which has left nearly seven million people dead, was the result of a mishap in a Chinese laboratory.
But four US intelligence agencies believe Covid occurred through natural transmission, while two others remain undecided, the Journal reported.
Shanghai Covid-19 lockdown - in pictures
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Nearly empty roads amid a phased lockdown due to Covid-19 in Shanghai, China. The country's largest city reported more than 13,000 daily Covid cases for the first time, as a sweeping lockdown of its 25 million residents and mass testing uncovered extensive spread of the highly infectious Omicron variant. Bloomberg -
Medical workers gather near a banner that reads 'Unite as one, resolutely win the battle against epidemic' during a departure ceremony before leaving for Shanghai, in Jinan, east China's Shandong Province. AP -
China has sent more than 10,000 health workers from across the country to Shanghai, including 2,000 military medical staff, as it struggles to tackle the rapidly spreading Covid-19 outbreak. AFP -
Workers in protective suits direct residents lining up for Covid testing. Reuters -
Medical workers and volunteers in a compound where residents are being tested for Covid-19. AFP -
Authorities are building the world’s largest makeshift isolation centre to help contain the outbreak in Shanghai. AFP -
A delivery man arranges his orders during lockdown in the Jingan district of western Shanghai. AP -
A worker in protective gear chats with residents outside shuttered shops in the Jingan district of western Shanghai. AP -
The number of cases continues to rise in Shanghai and Jilin, a north-eastern province. AP -
Residents wait to submit samples for tests at a community centre in the Pudong New Area of Shanghai. AP -
A person walks a dog. Reuters -
Policemen wearing protective gear on a street during patrols. AFP -
Workers remove their protective gear at the entrance of a neighbourhood. AFP -
Food shortages and shuttered manufacturing plants have hit residents hard. AFP -
Workers unload groceries from a truck before distributing them to residents. AP -
A man helps a woman to consume a packet of traditional Chinese medicine, as she sits by the side of a road outside a residential compound. Reuters -
Empty highways at night in Shanghai. Bloomberg
White House National Security Adviser Jake Sullivan stressed that 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," Mr Sullivan told CNN.
In mid-February, the World Health Organisation pledged to do everything possible "until we get the answer" on Covid's origins, denying a report saying it had abandoned its investigation.
The scientific community considers it crucial to determine the origins of the pandemic to better fight or even prevent the next one.
