US President Joe Biden has ordered intelligence officials to "redouble" efforts to investigate the origins of Covid-19, including the theory that it came from a laboratory in China.
He said the US intelligence community was split on whether it came from a lab accident or emerged from human contact with an infected animal.
Mr Biden asked the groups to report back to him within 90 days.
China has rejected the laboratory theory.
"Smear campaigns and blame shifting are making a comeback, and the conspiracy theory of 'lab leak' is resurfacing," its embassy in the US said in a statement on Thursday.
Since it was first detected in the Chinese city of Wuhan in late 2019, more than 168 million cases of Covid-19 have been confirmed worldwide and at least 3.5 million deaths reported.