The US on Wednesday, 9 March, said that Russia could be planning a chemical weapon attack on Ukraine.
"We should all be on the lookout," the White House added.
The statement came in the backdrop of Russia accusing the US of operating biowarfare laboratories in Ukraine.
White House Press Secretary Jen Psaki, in a series of tweets, called the Russian claim "preposterous".
She further added that it was actually Moscow "that has a long and well-documented track record of using chemical weapons".
The possibility of a false-flag attack was also raised.
Psaki said that the Russian accusation were an "obvious ploy" for premeditated and unprovoked attacks in the future.
"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," Psaki added.
Officials in western European countries shared similar concerns, stating the risk of excalation of war, including the possibility of the Russians using non-conventional weapons.
"We've got good reason to be concerned," one official said on the condition of anonymity, the BBC reported.
Psaki's comments about Moscow's "well-documented track record" were prpbably a reference to the Syrian War, in which Russia was an enabler of the Assad government's use of chemical weapons.
Russia had even diplomatically protected Syria at the Organisation for the Prohibition of Chemical Weapons and at the UN Security Council.
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