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 counterattack 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 politicised visit by a World Health Organisation (WHO) team in January also proved inconclusive, and faced criticism for lacking transparency and access.

The search for the origins of the COVID pandemic that has killed millions and crippled economies is at a standstill even as time is running out, scientists charged with the task by the UN warned Wednesday.

An initial report by the WHO team based on a January mission to Wuhan, China, ground zero of the global pandemic, concluded that the SARS-CoV-2 virus probably jumped from bats to humans via an intermediate animal.

A competing hypothesis that the virus leaked from a specialised virology lab in Wuhan was deemed "extremely unlikely”.

But in a comment in the journal Nature, the scientists said that mission was only intended as a "first step in a process that has stalled”.

"The search for the origins of SARS-CoV-2 is at a critical juncture,” they wrote.

"The window of opportunity for conducting this crucial enquiry is closing fast.”

Tracing the biological trail back to the earliest pockets of the disease becomes more difficult as evidence disappears or becomes corrupted.

The statement comes less than two weeks after the WHO, in a bid to revive the probe, urged China to hand over information on the earliest COVID-19 cases.

This should include COVID data for 174 infections identified in December 2019 that China failed to share during the initial investigation, the WHO experts said.