Russia faces 'catastrophic consequences' if it uses nuclear weapons in Ukraine: national security advisor

United States National Security Advisor Jake Sullivan said on CBS' Face the Nation on Sunday that Russian President Vladimir Putin will face "catastrophic consequences" if he follows through with this threat of using nuclear weapons in Ukraine once Russia's anticipated annexation of Eastern Ukrainian territories is completed this week.

The conversation took place shortly after Ukrainian President Volodymyr Zelenskyy revealed to correspondent Margaret Brennan that Putin's warnings should be taken seriously.

“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 explained to Brennan.

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"We have, in public, been equally clear, as a matter of principle, that the United States will respond decisively if Russia uses nuclear weapons and that we will continue to support Ukraine in its efforts to defend its country and defend its democracy," Sullivan added.

Sullivan also reflected upon the ongoing crisis at the Zaporizhzhia Nuclear Power Plant in Enerhodar, Ukraine, where shelling has damaged parts of Europe's largest fission station. Officials in Kyiv and Moscow have exchanged blame for the bombardments, which have forced a shutdown to avert a potential radioactive disaster that could dwarf the 1986 catastrophe at Chernobyl.

"It is actually still being operated by the Ukrainian operators who are essentially at gunpoint from the Russian occupying forces. And the Russians have been consistently implying that there may be some kind of accident at this plant," Sullivan explained.

"We've been working with the International Atomic Energy Agency and with Ukrainian energy regulators to try to make sure that there is no threat posed by a meltdown or something else from the plant. We will continue to do that," Sullivan stressed, "but it's something we all have to keep a close eye on."

READ MORE: Ukraine's Zaporizhzhia Nuclear Power Plant shuts down its last reactor amid mounting meltdown fears