It has been seven months since Russia launched its invasion of Ukraine in February, and in a rare, televised address on Wednesday, Russian President Vladimir Putin indirectly threatened that he is prepared to use nuclear weapons to escalate the war.

Zelenskyy said Russia is targeting nuclear power plants in Ukraine, so there could be some truth to Putin’s claims.

“He wants to scare the whole world. These are the first steps of his nuclear blackmail. I don’t think he’s bluffing,” Zelenskyy said.

U.S. National Security Advisor Jake Sullivan made clear that the United States is taking Putin’s nuclear threat seriously.

“We have communicated directly, privately, at very high levels to the Kremlin, that any use of nuclear weapons will be met with catastrophic consequences for Russia,” Sullivan said on “Face the Nation” Sunday morning.

Referendum voting in Russian-occupied territories of Ukraine is underway, according to Russian state media. Western and Ukrainian officials are rebuking them as a sham used by the Kremlin to try to legitimize Russia’s annexation of Ukrainian territory.

Zelenskyy said a lot of people will be forced to participate in these referenda, but that there is no support for it in society. He said a threat could emerge if the Russians complete the referendum and announce the results.

“This would make it impossible, in any case, to continue any diplomatic negotiations with the President of Russian Federation, and he knows it very well,” Zelenskyy said. “I think it’s a very dangerous signal from President Putin that tells us that Putin is not going to finish this war.”