Jake Sullivan: US will act ‘decisively’ if Russia uses nuclear weapons in Ukraine

America and its allies will act "decisively" if Russia uses a tactical nuclear weapon in Ukraine, US national security adviser Jake Sullivan said on Sunday, reaffirming the Joe Biden White House's previous response to mounting concerns that Vladimir Putin's threats are in increased danger of being realized.

"We have communicated directly, privately and at very high levels to the Kremlin that any use of nuclear weapons will be met with catastrophic consequences for Russia, that the US and our allies will respond decisively, and we have been clear and specific about what that will entail," Sullivan told CBS's Face The Nation.

Sullivan said that the Russian leader Putin had been "waving around the nuclear card at various points through this conflict", and it was a matter that Biden's administration has "to take deadly seriously because it is a matter of paramount seriousness - the possible use of nuclear weapons for the first time since the second world war".

In a separate interview with CBS, Ukraine president Volodymyr Zelenskiy said he was not certain that Putin was bluffing with nuclear threats. "Maybe yesterday it was bluff. Now, it could be a reality," he said. "He wants to scare the whole world."

The administration's security chief said that Russia's nuclear threat against Ukraine, including extending its nuclear umbrella over eastern parts of the country that are still being contested seven months after its invasion, would not deflect the US and its allies.

"We will continue to support Ukraine in its efforts to defend its country and defend its democracy," Sullivan said, pointing to more than $15bn in weapons, including air defense systems, hundreds of artillery pieces and rounds of artillery, that the US has supplied to Ukraine.

He said that Moscow's mobilization of troops was a "sham referenda in the occupied regions" that would not deter the US. "What Putin has done is not exactly a sign of strength or confidence - frankly, it's a sign that they're struggling badly on the Russian side," Sullivan said.