WASHINGTON — Ukrainian President Volodymyr Zelenskyy says he takes Russia’s nuclear threats seriously and does not believe Russian President Vladimir Putin is bluffing.

“Maybe yesterday it was a bluff. Now, it could be a reality,” Zelenskyy told CBS News in a Sunday interview.

“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, pointing to Russian shelling at Ukraine’s Zaporizhzhya nuclear power plant.

Zelenskyy’s remarks come as Moscow rushes to conduct referendums in occupied Ukrainian territory aimed at making them part of Russia.

Putin has previously said that Moscow would see Ukrainian attacks on these regions as attacks on Russian territory, and would use all means to defend it.

“If the territorial integrity of our country is threatened, we will absolutely use all available means to protect Russia and our people. This is not a bluff,” Putin said on Wednesday, announcing the mobilization of 300,000 reserve troops for battle in Ukraine.

Observers have widely understood this as a threat to use nuclear weapons.

U.S. national security adviser Jake Sullivan meanwhile said in a CBS interview on Sunday that any use of nuclear weapons would have catastrophic consequences for Russia and that the U.S. government had told the Kremlin this “directly, privately (and) at very high levels.”

Voting in internationally criticized referendums for four Russian-occupied Ukrainian territories to become part of Russia continued into its third day on Sunday despite shelling by Ukrainian forces.

The hastily convened polls were taking place in the eastern regions of Luhansk and Donetsk as well as in Zaporizhzhya and Kherson in the south, with authorities the Luhansk city of Alchevsk even saying voting could take place in bomb shelters.

According to the pro-Russian authorities, two people died in a hotel near Kherson in a missile attack.

Meanwhile, in the city of Enerhodar in the Zaporizhzhya region, a polling station had to be moved to another location due to massive shelling from the Ukrainian side, Russian state news agency Tass reported.

Neither Ukraine nor the international community recognizes the vote under Russia as the occupying power. They are not considered to be legitimate referendums because they are being held without the permission of Ukrainian authorities, under martial law and not according to democratic principles.

The voting is scheduled to run until Tuesday, with the population asked to vote “yes” or “no” on whether the territories should join the Russian Federation.

A positive result for the Kremlin, seen as a foregone conclusion, could see Putin annexing the regions as early as Friday.

Zelenskyy and the Group of Seven, the leading industrialized democracies, have described the referendums as a “sham.”

Western countries are preparing a new package of sanctions in response to the possible annexation. In 2014, Russia already annexed the Black Sea peninsula of Crimea.

Serbia’s government, despite its good relations with Moscow, has meanwhile joined the host of Western nations saying they will not recognize the results of the referendums in the Russianoccupied territories in Ukraine.

Recognition of these referendums “would completely violate our national and state interests, the preservation of sovereignty and territorial integrity and the inviolability of borders,” Serbia’s Foreign Minister Nikola Selakovic said at a news conference in Belgrade, Serbian media reported.

Selakovic was referring to what Belgrade sees as a parallel between Russia’s attack on Ukraine and Serbia’s own situation with the issue of Kosovan independence.

The former Serbian province of Kosovo declared its independence in 2008, a move still not recognized by Serbia, but by most other states.