Russian President Vladimir Putin has announced a partial military mobilization in Russia as his invasion of Ukraine nears seven months and Kyiv regains territory in its counteroffensive.

Putin also warned the West that "it's not a bluff" that Russia would use all the means at its disposal to protect its territory.

Putin's address, broadcast to the nation on September 21, comes a day after Russian-occupied regions in eastern and southern Ukraine announced plans to hold votes on being incorporated into Russia, triggering outrage and condemnations not only in Kyiv but from much of the international community.

Analysts say the Kremlin-backed efforts to swallow up four regions could set the stage for Moscow to escalate the war following Ukrainian successes on the battlefield.

The referendums, which have been expected to take place since the first months of the war, will start on September 23 in the Luhansk, Kherson, Zaporizhzhya, and Donetsk regions.

In his address, Putin accused the West of engaging in "nuclear blackmail" and claimed, without providing proof, that "high-ranking representatives of the leading NATO states" had allegedly made statements "about the possibility of using nuclear weapons of mass destruction against Russia."

"To those who allow themselves such statements regarding Russia, I want to remind you that our country also has various means of destruction, and for separate components and more modern than those of NATO countries. When the territorial integrity of our country is threatened, to protect Russia and our people, we will certainly use all the means at our disposal," Putin said, adding, "It's not a bluff."

Putin said the partial mobilization was due to start immediately on September 21.

"We are talking about partial mobilization, that is only citizens who are currently in the reserve will be subject to conscription, and above all those who served in the armed forces have a certain military specialty and relevant experience," Putin said.

Putin said his aim was to "liberate" east Ukraine's Donbas region, claiming without providing any proof that most people in the region did not want to return to what he called the "yoke" of Ukraine.

Russian Defense Minister Sergei Shoigu said in a televised interview on September 21 that only those with relevant combat and service experience would be mobilized and not conscripts or students.

He said 5,937 Russian troops have been killed in Ukraine so far, a fraction of Western and Ukrainian estimates of Russian military losses.

Shoigu's casualty update on Russian losses is the third such public release. The last update came in late March, when the Defense Ministry claimed 1,351 Russian soldiers had been killed in Ukraine.

British Defense Secretary Ben Wallace described Putin's mobilization announcement as "an admission that his invasion is failing."

"He and his defense minister have sent tens of thousands of their own citizens to their deaths, ill-equipped and badly led," Wallace said in a statement. "No amount of threats and propaganda can hide the fact that Ukraine is winning this war, the international community are united, and Russia is becoming a global pariah."

Dutch Prime Minister Mark Rutte said Putin's mobilization order was a sign of panic at the Kremlin that should not be taken as a direct threat of full-out war with the West.

"The mobilization, calling for referenda in the Donetsk -- it is all a sign of panic. His rhetoric on nuclear weapons is something we have heard many times before, and it leaves us cold," Rutte told Dutch broadcaster NOS.

Putin's address was broadcast a day after plans were announced in four Ukrainian regions occupied by Russian forces to hold votes on being incorporated into Russia, a move that was widely condemned by the international community.

U.S. national-security adviser Jake Sullivan said Washington rejected any such referendums "unequivocally," while German Chancellor Olaf Scholz and French President Emmanuel Macron both used the word "sham" to describe the planned action.

"This has no legal standing," Macron said on September 20 before addressing the UN General Assembly in New York. "The very idea of organizing referendums in territories that have experienced war...is the sign of cynicism."

"It is very, very clear that these sham referendums cannot be accepted and are not covered by international law," Scholz told reporters as he attended the UN General Assembly.

Canadian Prime Minister Justin Trudeau also denounced Russia's planned referendums, saying Canada would never recognize them.

"This is a blatant violation of international law. It is a further escalation of war. And it is unacceptable," he said on Twitter.

The European Union strongly condemned the planned referendums and threatened additional sanctions.

"Russia, its political leadership, and all those involved in these 'referenda' and other violations of international law in Ukraine will be held accountable, and additional restrictive measures against Russia would be considered," EU foreign policy chief Josep Borrell said in a statement.

He said the votes cannot be considered "as the free expression of the will of the people" in these regions.

Ukrainian President Volodymyr Zelenskiy hailed the Western condemnation of the referendums.

"I thank all the friends and partners of Ukraine for their massive and firm condemnation of Russia's intentions to organize yet more pseudo-referendums," he said in his nightly video address.

"The situation on the front line clearly indicates that the initiative belongs to Ukraine," he said. "Our positions do not change because of the noise or any announcements somewhere. And we enjoy the full support of our partners in this."

Ukrainian Foreign Minister Dmytro Kuleba said earlier that the referendums will "not change anything," adding on Twitter that "Russia has been and remains an aggressor illegally occupying parts of Ukrainian land. Ukraine has "every right to liberate its territories and will keep liberating them whatever Russia has to say."

Former Russian President Dmitry Medvedev, who is currently the deputy chief of Russia's Security Council, wrote on Telegram on September 20 that the referendums on Ukraine's occupied territories are "important" to "reinstate the historic justice."

"After [the referendums] are conducted and the new territories become part of Russia, the geopolitical transformation in the world will take on an irreversible character," Medvedev wrote, adding that attempts to encroach Russia's territory is a crime and Russia could use "all means of self-defense" after the Ukrainian territories become part of Russia.

With reporting by dpa, AFP, Reuters, and AP