Asked about the threat, Zelensky said "all of the world" should be worried that Russia "began to speak about... nuclear weapons or some chemical weapons."

"They could do it, I mean they can," he told CNN. "For them, life of the people is nothing... let's not be afraid -- be ready."

Burns said Thursday that Russia's battlefield setbacks raised the risk that President Vladimir Putin could deploy a tactical or low-yield nuclear weapon.

The Kremlin said it had placed Russian nuclear forces on high alert shortly after the assault began February 24, but the United States says it has not seen any sign of unusual nuclear movements.

Russian military doctrine includes the "escalate to de-escalate," principle of launching a small nuclear weapon to regain the initiative in war.

US President Joe Biden is "deeply concerned about avoiding a third world war, about avoiding a threshold in which nuclear conflict becomes possible," said Burns.