China on Wednesday accused the U.S. of "spreading conspiracy theories and disinformation" as the theory resurfaced that the coronavirus emerged from a Wuhan laboratory, while urging Washington to open its virology facilities to scrutiny.

Led by the U.S., pressure is mounting for a new probe into the origins of COVID-19 after a World Health Organization (WHO) mission to China, beset by delays and dogged by political baggage, returned inconclusive findings.

The Wuhan Institute of Virology has returned to the spotlight after a report in The Wall Street Journal, citing U.S. intelligence, said three lab workers there were hospitalised in November 2019 with coronavirus-like symptoms, a month before the pandemic's first declared case.

The newspaper also said researchers had collected samples seven years earlier from a mine in southwestern China, where miners had contracted a mysterious illness from a new, bat-born coronavirus.

Repeating an earlier denial of the report, Zhao Lijian, spokesman for China's foreign ministry, accused the U.S. of "spreading conspiracy theories and disinformation such as a laboratory leak".

The theory that the killer virus leaked from a Chinese lab was originally fuelled by, among others, the administration of former President Donald Trump.

Zhao said it was "disrespectful" to the WHO probe to revive the theory, and risked an "undermining of global solidarity to fight the virus."

Since infecting its first victims in the central Chinese city of Wuhan in late 2019, the pathogen has struck almost every country in the world, killing more than 3.4 million people and eviscerating national economies.

"If the U.S. really wants full transparency then it should, like China did, invite WHO experts to visit the U.S. and investigate," Zhao added.

"Open up Fort Detrick military base as early as possible, and all the bio labs the U.S. has around the world," he said, referring to a U.S. research facility in Maryland.

The long-delayed report, by the team of international experts sent to Wuhan and their Chinese counterparts, drew no firm conclusions on the origins of the pandemic.

Instead, the WHO-led team said the virus most likely jumped from bats to humans via an intermediate animal.

Initially, the expert report said a theory involving the virus leaking from a lab was "extremely unlikely."

After the report was released, however, WHO chief Tedros Adhanom Ghebreyesus insisted all theories remained on the table.