David Stilwell, the top U.S. diplomat for Asia, issued 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 in 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 city of Wuhan, where cases were first reported in late 2019.