Joe Biden has said he doubts Russian President Vladimir Putin would use a tactical nuclear weapon - as Ukraine pleaded for a rapid increase in Western military aid to defend against missile strikes on its cities.
Putin, under domestic pressure to ramp up the war as his forces have lost ground since early September, ordered missile strikes on Monday in response to an alleged Ukrainian attack on Russia's bridge to annexed Crimea last weekend.
In recent weeks, Moscow moved to annex new tracts of Ukraine after referendums widely denounced as illegal, mobilised hundreds of thousands of Russians to fight, and repeatedly threatened to use nuclear arms, stoking alarm in the West.
In a CNN interview on Wednesday, Mr Biden said Putin is a "rational actor who has miscalculated significantly”.
Asked how realistic he believed it would be for the Russian President to use a tactical nuclear weapon, Mr Biden responded: "Well, I don’t think he will."
His comments come after he last week said the risk of nuclear “Armageddon” was at the highest level since the 1962 Cuban Missile Crisis.