They're pursuing rival theories including the possibility of an accident at a virology lab in Wuhan, and are likely to release the full report to the public once it's completed.
The Chinese government has repeatedly rejected any link between COVID-19 and the virology lab.
Ministry of Foreign Affairs spokesman Zhao Lijian is accusing the US of trying to make China a scapegoat.
"Mandarin, then translation: "The US side does not care about facts and truth, nor is it interested in serious scientific origin tracing, but wants to use the pandemic to engage in stigmatisation and political manipulation, and to shirk its responsibility. This is disrespectful of science and irresponsible to people's lives, and moreover, it undermines the global anti-epidemic efforts and solidarity."
In the United Kingdom, Boris Johnson is denying claims from his former top advisor that the government's handling of the COVID-19 caused thousands of needless deaths.
Cases of the variant first detected in India have almost doubled in the U-K since last week to nearly 7,000.
India continues to record the highest daily increase in COVID-19 cases around of the world, with its overall total now at 27.37 million.
Prime Minister Narendra Modi's government has faced mounting criticism for its failure to secure vaccines, as the devastating second wave causes India's death toll to rise above 315,000.
Africa is also facing vaccine shortages, with the World Health Organisation saying the continent needs at least 20 million doses of the AstraZeneca jab within six weeks if those who've had their first shot are to get the second in time for maximum protection.
People in Australia must stay at least 1.5 metres away from others. Find out what restrictions are in place for your state or territory.
Testing for coronavirus is now widely available across Australia. If you are experiencing cold or flu symptoms, arrange a test by calling your doctor or contact the Coronavirus Health Information Hotline on 1800 020 080.
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