US President Joe Biden has said risk of a nuclear "Armageddon" is at its highest level since the 1962 Cuban Missile Crisis.
Mr Biden said Russia's President Vladimir Putin was "not joking" when he spoke of using tactical nuclear weapons after suffering setbacks in Ukraine, reports BBC.
The US was "trying to figure out" Mr Putin's way out of the war, he added.
The US and the EU have previously said Mr Putin's nuclear sabre-rattling should be taken seriously.
However, the US National Security Adviser Jake Sullivan last week said that, despite Moscow's nuclear hints, the US had seen no signs that Russia was imminently preparing to use a nuclear weapon.
Bd-pratidin English/Tanvir Raihan
