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White House press secretary Jen Psaki on Wednesday warned of the potential for Russia to use chemical weapons in Ukraine after Moscow alleged the United States was housing biological weapons in Ukrainian territory.

Psaki called the claim from Russia “false” and “preposterous,” and she warned it could serve as a pretext for the Russians to deploy chemical weapons in their assault on Ukraine.

“It’s the kind of disinformation operation we’ve seen repeatedly from the Russians over the years in Ukraine and in other countries, which have been debunked, and an example of the types of false pretexts we have been warning the Russians would invent,” Psaki said in a statement.

“Also, Russia has a track record of accusing the West of the very violations that Russia itself is perpetrating. In December, Russia falsely accused the U.S. of deploying contractors with chemical weapons in Ukraine,” she continued.

“This is all an obvious ploy by Russia to try to try to justify its further premeditated, unprovoked, and unjustified attack on Ukraine. Now that Russia has made these false claims, and China has seemingly endorsed this propaganda, we should all be on the lookout for Russia to possibly use chemical or biological weapons in Ukraine, or to create a false flag operation using them. It’s a clear pattern,” she said.

Maria Zakharova, a Russian Foreign Ministry spokeswoman, claimed Russia discovered evidence of a program to develop anthrax and other biological weapons run by the United States in Ukraine, Reuters reported. She alleged the program was backed by the Pentagon.

“This Russian disinformation is total nonsense and not the first time Russia has invented such false claims against another country,” State Department spokesman Ned Price said in a statement. “Also, these claims have been debunked conclusively and repeatedly over many years.”

Pentagon press secretary John Kirby at a Wednesday briefing called the claims “absurd” and “laughable.”

Warnings from the White House that Russia could use chemical weapons in its invasion of Ukraine come as the Russian military is increasingly targeting civilians and nongovernment buildings.

Ukrainian officials said a Russian strike earlier Wednesday hit a hospital building in the city of Mariupol, including a maternity ward. Videos have circulated of women and children attempting to flee the violence, and hundreds of civilians have died since the invasion began last month.