Russia-Ukraine updates: Putin suspends key US-Russia nuclear treaty in speech denouncing West

Russia began holding its "sham" referendums in four Ukrainian regions it occupies on Friday, asking people to vote on whether they want to join Russia in an effort to legitimize its annexation of the regions.

The referendums are being held in Donetsk and Lugansk in the Donbas region and occupied territory in the southern Zaporizhzhia and Kherson regions.

The referendums, announced three days ago, will be held for five days, with in person voting taking place on Tuesday. The majority of voting will be done at peoples' homes or remotely.

Russia had previously done this in Crimea in 2014, but this vote is expected to have even less legitimacy.

Western countries have already rejected the referendums as illegal shams and only a tiny handful of authoritarian countries are likely to recognize them.

The United States has been sending private warnings to Moscow about the consequences of using nuclear weapons, a U.S. official told ABC News.

President Joe Biden has also made the warnings publicly, most recently in his address to the U.N. General Assembly on Thursday.

The warnings have been vague, a deliberate strategy designed to keep Kremlin officials guessing on what the U.S. response would actually be in the event of a nuclear strike, according to The Washington Post, which was the first to report on the private warnings.

It is not clear who has been delivering the messages to Moscow, or whether a message was sent after Russian President Vladimir Putin's most recent nuclear threat.

-ABC News' Sarah Kolinovsky

Ukrainian President Volodymyr Zelenskyy spoke directly to Russian citizens in his latest nightly address in response to President Vladimir Putin's partial mobilization of troops to fight in Ukraine.

Switching from Ukrainian to speak in Russian, he remarked that people are protesting the war across Russia because they "understand that they were simply thrown -- thrown to [their] death."

To those who are silent, "You are accomplices in all these crimes, murders and torture of Ukrainians," he said, wearing a black T-shirt that said in English: "We Stand with Ukraine."

Russians options to survive, he said, are to "protest, fight, run away or surrender to Ukrainian captivity."

In an address to the United Nations Security Council on Thursday, Russian Foreign Minister Sergei Lavrov fought back against what he described as a "propaganda operation" by Ukraine and its Western allies to change the "narrative" in the war.

“There’s an attempt today to impose on us a completely different narrative about a Russian aggression as the origin of all the tragedy," Lavrov said.

He alleged that such a move comes after eight years of Ukrainian forces killing the inhabitants of the Russian-backed Dunbas region of eastern Ukraine "with impunity."

Lavrov's address to the Security Council came shortly after U.S. Secretary of State Antony Blinken called planned referendums to allow residents of the Dunbas and other areas under Russian control to vote on joining the Russian Federation a "sham." Blinken said it is part of a "diabolical" Kremlin plan to annex more Ukrainian territory.

Lavrov also accused Ukraine of treading on the "rights and freedoms" of residents in the Dunbas, including the right to speak Russian.

“They declared all those who don’t agree there as terrorists and for eight years the Kiev regime has been conducting a military operation against the peaceful civilians," Lavrov said.

He then accused Ukraine's Western allies, including the United States, of being a "party to the conflict" by supplying Ukraine with weapons.

“Their goal is obvious. They are clearly stating (it is) to drag out the fighting as long as possible in spite of the victims and destruction, in order to wear down and weaken Russia," Lavrov said.

"The intentional fomenting of this conflict by the collective West remains unpunished," Lavrov said. "Of course, you won’t punish yourselves."