The study, published this week by the US National Institutes for Health (NIH), showed that at least seven people in five different US states were infected with SARS-CoV-2, the virus that causes Covid-19, weeks before the first official cases were reported.

Zeng Guang, chief epidemiologist with the Chinese Center for Disease Control and Prevention, told state-owned tabloid the Global Times that attention should now shift to the US, which was slow to test people in the early stages of the outbreak, and is also the home of many biological laboratories.

“All bio-weapons related subjects that the country has should be subject to scrutiny,” he was quoted as saying.

Commenting on the study on Wednesday, foreign ministry spokesman Zhao Lijian said it was now “obvious” that the Covid-19 outbreak had “multiple origins” and that other countries should cooperate with the World Health Organization (WHO).

The origin of the pandemic has become a source of political tension between China and the US, with much of the focus on the Wuhan Institute of Virology, located in the city where the outbreak was first identified in late 2019.

A report by a US government national laboratory concluded that it was plausible that the virus had leaked from the Wuhan lab, the Wall Street Journal reported earlier this month.

A previous study has raised the possibility that SARS-CoV-2 could have been circulating in Europe as early as September, but experts said this didn’t necessarily mean it did not originate in China, where many SARS-like coronaviruses have been found in the wild.