Russia could be planning a chemical or biological weapon attack in Ukraine - and "we should all be on the lookout", the White House has said, BBC reported.
Press secretary Jen Psaki said Russia's claims about US biological weapon labs, and chemical weapon development in Ukraine, were preposterous.
She called the false claims an "obvious ploy" to try to justify further premeditated, unprovoked attacks.
It comes after Western officials shared similar concerns about fresh attacks.
They said they were "very concerned" about the risk the war could escalate, and particularly the possibility of Russia using non-conventional weapons.
This most likely refers to chemical weapons although the term also covers tactical (small-scale) nuclear weapons, biological weapons and dirty bombs, according to BBC.
"We've got good reason to be concerned," said one Western official.
They said this was partly because of what had been seen in other places where Russia has been engaged - notably Syria where chemical weapons were used by its allies.
Ms Psaki said: "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."
Earlier on Wednesday, the UK Ministry of Defence said in a tweet that Russia had used thermobaric rockets in Ukraine. These rockets are also known as vacuum bombs because they suck in oxygen from the surrounding air to generate a high-temperature explosion.
This makes them more devastating than conventional explosives of a similar size, and can have a terrible impact on people caught in their blast radius, BBC reported.
The fears around chemical weapon use were borne from Russian claims potentially "setting the scene" for some kind of "false flag" claim, Western officials said.
In a tweet, the Russian Embassy referred to claims that "recently found documents" showed components of biological weapons were made in Ukrainian laboratories - with funding from the US Department of Defense.
The US dismissed the claims, saying it was the "kind of disinformation operation we've seen repeatedly from the Russians over the years in Ukraine and in other countries".
Russian officials and media have also claimed in recent days that Ukraine was planning to build a so-called dirty bomb - which disperses radioactive material.
Meanwhile Russia's foreign minister has claimed Ukraine had been seeking nuclear weapons, according to BBC.
Some believe Moscow is pushing these claims to come up with a justification for its own public about why it invaded Ukraine. But Western officials also fear they could be used as the basis for a "false flag" event.
