A classified US intelligence report delivered to the White House on Tuesday was inconclusive on the origins of the Covid-19 pandemic, in part due to a lack of information from China, according to US media reports.

The assessment, ordered by President Joe Biden 90 days ago, was unable to definitively conclude whether the virus that first emerged in central China had jumped to humans via animals or escaped a highly secure research facility in Wuhan, two US officials familiar with the matter told the Washington Post.

They said parts of the report could be declassified in the coming days.

The debate over the origins of the virus that has killed more than 4 million people and paralysed economies worldwide has become increasingly contentious.

When Biden assigned the investigation, he said US intelligence agencies were split over the "two likely scenarios" -- animals or lab.

Former president Donald Trump and his aides had helped fuel the lab-leak theory, using it to deflect blame for their administration's handling of the world's biggest outbreak, and instead finger point at Beijing, which strongly denies the hypothesis.

China on Wednesday urged the WHO to visit the US military biolab Fort Detrick, after rejecting its calls for a second stage of the Covid origins probe focusing on Chinese laboratories last month.

"If (the US) want to baselessly accuse China, they better be prepared to accept a counter-attack from China," Fu Cong, head of the foreign ministry's arms control department, told reporters.

"If the US thinks China is guilty, they need to come up with evidence to prove that China is guilty. You don't blame a victim for not providing information to incriminate himself."

Despite Biden's directive that the intelligence community "redouble their efforts" to untangle the origin debate, the 90-day review brought them no closer to consensus, the officials told the Post.

Beijing has rejected calls from the US and other countries for a renewed origin probe after a heavily politicized visit by a World Health Organization team in January also proved inconclusive, and faced criticism for lacking transparency and access.

Pressure has meanwhile increased to evaluate the lab-leak theory more thoroughly.

At the outset of the pandemic, the natural origin hypothesis -- that the virus emerged in bats then passed to humans, likely via an intermediary species -- was widely accepted. But as time has worn on, scientists have not found a virus in either bats or another animal that matches the genetic signature of SARS-CoV-2.

In the face of China's reluctance to open up to outside investigators, experts are increasingly open to considering the theory that the virus might have leaked out of a lab conducting bat coronavirus research in Wuhan, an idea once dismissed as a conspiracy propagated by the US far-right.