Washington, Mar 22 (IANS): Russias false accusation that Ukraine has biological and chemical weapons is a "clear sign" that a desperate President Vladimir Putin is considering using them himself, US President Joe Biden said.

The US President said Putin's "back is against the wall and now he's talking about new false flags he's setting up including, asserting that we in America have biological as well as chemical weapons in Europe – simply not true. I guarantee you", the Guardian reported.

"They are also suggesting that Ukraine has biological and chemical weapons in Ukraine. That's a clear sign he's considering using both of those. He's already used chemical weapons in the past, and we should be careful of what's about to come."

Putin "knows there'll be severe consequences because of the united Nato front," he said, without specifying what actions the alliance would take, the Guardian reported.

The remarks echo previous comments by officials in Washington and allied countries, who have accused Russia of spreading an unproven claim that Ukraine had a biological weapons programme as a possible prelude to potentially launching its own biological or chemical attacks.

Biden spoke after the Pentagon said it had seen "clear evidence" Russian forces were committing war crimes and that it was helping collect evidence.