The Ukrainian President is “really worried” Russia is planning to use chemical weapons in its ongoing invasion.
It comes after Russia claimed the US was helping Ukraine make chemicals weapons – something the White House dismissed as laughable.
Russia has requested a meeting of the United Nations security council to discuss its accusations.
White House Press Secretary Jen Psaki said the accusations were “preposterous” and “laughable”.
“This is all an obvious ploy by Russia to try to justify its further pre-meditated, unprovoked, and unjustified attack on Ukraine,” she said.
Olivia Dalton, spokesperson for the US Mission to the United Nations, said: “This is exactly the kind of false flag effort we have warned Russia might initiate to justify a biological or chemical weapons attack.
“We're not going to let Russia gaslight the world or use the UN Security Council as a venue for promoting their disinformation.”
Meanwhile, Ukraine President Volodymyr Zelenskyy said he was “really, really worried” about the claims.
“We've been repeatedly convinced: if you want to know Russia's plans, look at what Russia accuses others of,” he said.
He said Ukrainians were “decent people” who would never consider using chemical weapons.
“I am the president of a decent country, a decent nation, and the father of two children,” he said. “No chemical or any other weapons of mass destruction were developed on my land.
“The whole world knows that. You [Russia] know that, and if you do something like that against us, you will get the most severe sanctions response.”
Russia has made its allegations a number of times in recent days – most recently through its foreign ministry and mission to the UN.
Russia's defence ministry published a briefing accusing the US Defence Threat Reduction Agency of laying a “leading role” in “financing and conducting military biological research on the territory of Ukraine”
Ukraine does have public health labs researching ways to fight dangerous diseases, with the support of the US, Europe and the World Health Organisation.
President Zelenskyy noted that the labs were used for “ordinary science - not military technology”.