The United States on Friday summoned China’s ambassador after a senior official in Beijing tweeted the “ridiculous” suggestion that the US military started the COVID-19 pandemic, the State Department said.

David Stilwell, the top US diplomat for Asia, gave a “stern representation” to Ambassador Cui Tiankai a day after foreign ministry spokesman Zhao Lijian tweeted the conspiracy theory.

“China is seeking to deflect criticism for its role in starting a global pandemic and not telling the world,” a State Department official said.

“Spreading conspiracy theories is dangerous and ridiculous. We wanted to put the government on notice we won’t tolerate it, for the good of the Chinese people and the world,” the official said.

Zhao, in tweets both Mandarin and English that gained wide traction on Chinese social media, a day earlier suggested that “patient zero” in the global pandemic may have come from the United States—not the Chinese metropolis of Wuhan, where cases were first reported in late 2019.

“It might be US army who brought the epidemic to Wuhan. Be transparent! Make public your data! US owe us an explanation,” tweeted Zhao, who is known for his provocative statements on social media.

Scientists widely believe that the pandemic started in a market in Wuhan that sold exotic animals for consumption.

As of 1700 GMT on Friday, more than 5,300 people have died around the world and more than 140,000 infected.

Cui, in contrast to Zhao, is known for his diplomatic approach and has publicly called for US-Chinese cooperation against the pandemic.