Joe Biden's top coronavirus adviser says he's "not convinced" the disease developed naturally and has called for further investigation into its origin.
Dr Anthony Fauci said the probe into "what went on in China" needs to continue until investigators "find out to the best of our ability what happened".
Earlier this month, a group of leading scientists said the origin of the virus is still unclear and the theory that it was caused by a leak in a laboratory needs to be taken seriously until a rigorous data-led investigation proves it wrong.
Covid-19 emerged in the Chinese city of Wuhan in late 2019 and rapidly spread around the world, infecting more than 167 million people and killing almost 3.5 million.