The State Department summoned China’s ambassador to the United States on Friday morning after a spokesman for the Chinese Foreign Ministry accused the U.S. military of starting the coronavirus outbreak in Wuhan.

Ambassador Cui Tiankai refused to comment on his conversation with David Stilwell, the assistant secretary for East Asian and Pacific Affairs, as Cui was leaving the State Department about 11 a.m. He ignored questions shouted by three reporters who followed him as he walked from the building’s front door and got into the back of a car.

A State Department official said Stilwell gave a very “stern representation” of the facts to the Chinese ambassador, who was described as “very defensive.”

Lijian Zhao suggested on Thursday that the U.S. Army was responsible for originating the infection in Wuhan.

“When did patient zero begin in US?” Lijian tweeted Thursday. “It might be US army who brought the epidemic to Wuhan.”

The State Department decided to confront the Chinese directly over what officials said was a growing disinformation campaign. Morgan Ortagus, a State Department spokeswoman, said China is seeking to deflect blame for its role in “starting a global pandemic and not telling the world.”

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

The Chinese have complained that Secretary of State Mike Pompeo and other administration officials and allies have referred to the virus as the “Wuhan virus,” in an effort to emphasize that its origin is foreign. China has called the word choice “slander.” The disease caused by the virus is known as covid-19.

The State Department has been increasingly aggressive at countering claims by China and Iran, which also is struggling with an outbreak that has killed at least 500 people, the third highest number of fatalities after China and Italy.

On Thursday, Iranian Foreign Minister Mohammad Javad Zarif charged that U.S. sanctions against Iran have caused medical shortages. Ortagus tweeted that Iran should release American and other foreign prisoners held unjustly amid fears the virus is spreading in Iranian prisons. She also rejected Zarif’s charge that sanctions were responsible for Iran’s medical shortages.