President Joe Biden and other Group of 7 leaders condemned Russia's "sham referenda" in occupied Ukrainian territories, calling it a Russian attempt to "create a phony pretext for changing the status of Ukrainian sovereign territory."

“These actions clearly breach the United Nations Charter and international law and go diametrically against the rule of law among nations,” the leaders said in a joint statement.

The G7 also pledged to "never recognize these referenda which appear to be a step toward Russian annexation and we will never recognize a purported annexation if it occurs."

They added, "These sham referenda initiated today by Russia and its proxies have no legal effect or legitimacy, as demonstrated by Russia’s hasty methods of organization, which in no way respect democratic norms, and its blatant intimidation of local populations."

The group warned that it will stand ready to impose "further economic costs on Russia, and on individuals and entities — inside and outside of Russia—providing political or economic support for Russia’s illegal attempts to change the status of Ukrainian territory."

“In addition, we deplore deliberate Russian escalatory steps, including the partial mobilization of reservists and irresponsible nuclear rhetoric," the group said.

-ABC News' Ben Gittleston

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."