As Beijing grapples with a worsening outbreak of the Delta variant, an outlandish conspiracy theory linking the origin of the coronavirus to the United States military has gained renewed traction in China.

The wholly unfounded theory, which claims the virus may have been leaked from a US Army lab, has been repeatedly promoted by Chinese officials and state media since March last year.

But over the past week, Beijing has doubled down on the conspiracy, mobilizing its diplomats and vast propaganda apparatus to call for a World Health Organization (WHO) investigation into the US Army Medical Research Institute of Infectious Diseases at Fort Detrick, Maryland.

The campaign comes after Beijing rejected WHO's proposal for a second-phase probe into the origins of Covid-19 last month. The study would include audits of laboratories and markets in Wuhan, the original epicenter of the pandemic. That has drawn the ire of Beijing, with a top Chinese health official accusing WHO of "disregarding common sense and defying science."

WHO released an initial report from its Covid origins study in China in March, concluding the lab leak theory was "extremely unlikely." But a growing number of Western nations and scientists have questioned the thoroughness of the original report, accusing China of "withholding access to complete, original data and samples."