US President Joe Biden has ordered intelligence agencies to investigate the emergence of Covid-19, amid growing controversy about the virus's origins.
In a statement, Biden asked US intelligence groups to "redouble their efforts" and report to him within 90 days.
Covid-19 was first detected in the Chinese city of Wuhan in late 2019.
More than 168 million cases have since been confirmed worldwide and at least 3.5 million deaths reported.
Authorities linked early Covid-19 cases to a seafood market in Wuhan, and scientists believed the virus first passed to humans from animals.
But recent US media reports suggest growing evidence the virus could instead have emerged from a laboratory in China.
Beijing has condemned the reports and suggested it could have come from a US laboratory instead.
In a White House statement released on Wednesday, President Biden said he had asked for a report on the origins of Covid-19 after taking office, "including whether it emerged from human contact with an infected animal or from a laboratory accident".
On receiving it this month, he asked for "additional follow-up".
"As of today, the US Intelligence Community has coalesced around two likely scenarios but has not reached a definitive conclusion on this question," Biden said.
"Here is their current position: 'While two elements in the IC leans toward the former scenario and one leans more toward the latter - each with low or moderate confidence - the majority of elements do not believe there is sufficient information to assess one to be more likely than the other.'"
The president has now asked agencies to "redouble their efforts to collect and analyse information that could bring us closer to a definitive conclusion", and report to him within 90 days. He asked that Congress be kept "fully apprised" of the work.
He concluded by saying the US would "keep working with like-minded partners around the world to press China to participate in a full, transparent, evidence-based international investigation and to provide access to all relevant data and evidence".
In March this year, the World Health Organization (WHO) issued a report written jointly with Chinese scientists on the origins of Covid-19, saying the chances of it having started in a lab were "extremely unlikely". The WHO acknowledged further study was needed.
But questions have persisted and recent reports attributed to US intelligence sources say three members of the Wuhan Institute of Virology were admitted to hospital in November 2019, several weeks before China acknowledged the first case of the new disease in the community.
Anthony Fauci, President Biden's chief medical adviser, has maintained he believes the virus was passed from animals to humans, though he conceded this month he was no longer confident Covid-19 had developed naturally.
Biden's statement comes the day after Xavier Becerra, US secretary for health and human services, urged the WHO to ensure a "transparent" investigation into the virus's origins.
"The Covid-19 pandemic not only stole a year from our lives, it stole millions of lives," Becerra said in an address to the World Health Assembly, a conference organised by the WHO.
"Phase 2 of the Covid origins study must be launched with terms of reference that are transparent, science-based and give international experts the independence to fully assess the source of the virus and the early days of the outbreak."
The laboratory leak allegations were widely dismissed last year as a fringe conspiracy theory, after then-President Donald Trump said Covid-19 had originated from the Wuhan Institute of Virology. Many US media outlets described such claims as debunked or false.
On Tuesday, Trump sought to take credit in an emailed statement to the New York Post. "To me it was obvious from the beginning but I was badly criticised, as usual," he said. "Now they are all saying: 'He was right.'"