President Vladimir Putin ordered a partial military mobilization and vowed on Wednesday to use "all available means" to protect Russian territory, after Moscow-held regions of Ukraine suddenly announced annexation referendums.

The votes, already denounced by Kyiv and the West as a "sham", will dramatically up the stakes in the seven-month-old conflict in Ukraine by giving Moscow the ability to accuse Ukrainian forces of attacking Russian territory.

Four Russian-occupied regions of Ukraine — Donetsk and Lugansk in the east and Kherson and Zaporizhzhia in the south — said on Tuesday that they would hold the votes over five days beginning Friday.

In a pre-recorded address to the nation early on Wednesday, Putin accused the West of trying to "destroy" his country through its backing of Kyiv, and said Russia needed to support those in Ukraine who wanted to "determine their own future".

The Russian leader announced a partial military mobilization, with Defense Minister Sergei Shoigu telling state television that some 300,000 reservists would be called up.

'Not a bluff'

"When the territorial integrity of our country is threatened, we will certainly use all the means at our disposal to protect Russia and our people. This is not a bluff," Putin said.

Germany branded the partial call-up as a "wrong step" while jailed Kremlin critic Alexei Navalny said it would result in a "massive tragedy, in a massive amount of deaths".

Putin said that through its support for Ukraine the West was trying to "weaken, divide and ultimately destroy our country", while Shoigu said Moscow was "fighting not so much Ukraine as the collective West" in Ukraine.

The sudden flurry of moves by Moscow this week came with Russian forces in Ukraine facing their biggest challenge since the start of the conflict.

A sweeping Ukrainian counter-offensive in recent weeks has seen Kyiv's forces retake hundreds of towns and villages that had been controlled by Russia for months.

In a rare admission of military losses from Moscow, Shoigu said Wednesday 5,937 Russian soldiers had died in Ukraine since the launch of the military intervention in February.

'Wake-up, finally'

As Putin made his announcement, residents clearing rubble and broken glass from a nine-story apartment block hit by an overnight missile strike in the eastern Ukrainian city of Kharkiv.

Svetlana, 63, gathered with friends to look on as neighbors and municipal workers cleared debris, urged the region's Russian neighbors to ignore the mobilization and "to wake up, finally."

Meanwhile her neighbor, 50-year-old Galina, expressed bewilderment at Moscow's aims against Ukraine.

"They want to liberate us from what? From our homes? From our relatives? From friends? What else? From over life? They want to free us from being alive?" she told AFP.

The referendums follow a pattern first established in 2014, when Russia annexed the Crimea peninsula from Ukraine after a similar vote.

Like in 2014, Washington, Berlin, and Paris denounced the latest referendums and said the international community would never recognixe the results.

German Chancellor Olaf Scholz said they were a "sham", French President Emmanuel Macron called them a "travesty", and White House National Security Advisor Jake Sullivan said they were "an affront to the principles of sovereignty and territorial integrity".

"Sham referenda and mobilization are signs of weakness, of Russian failure," the US ambassador in Ukraine, Bridget Brink, said on Twitter.

"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," Ukrainian President Volodymyr Zelensky said in response.

Strike at nuclear plant

Kyiv said the referendums were meaningless and vowed to "eliminate" threats posed by Russia, saying its forces would keep retaking territory regardless of what Moscow or its proxies announced.

Political analyst Tatiana Stanovaya said the vote announcements were a direct result of the success of Ukraine's eastern counter-offensive.

"Putin does not want to win this war on the battlefield. Putin wants to force Kyiv to surrender without a fight," she said.

The Ukrainian nuclear operator Energoatom meanwhile on Wednesday accused Russia of again striking the Zaporizhzhia atomic power plant in southern Ukraine.

The strike damaged a power line causing the stoppage of several transformers of the number six reactor of the plant and forcing a brief start of emergency generators, Energoatom said.

"Even the presence of inspectors from the International Atomic Energy Agency does not stop" the Russians, it said, calling on the agency to "more resolute actions" against Moscow.

Europe's largest nuclear facility, located in Russian-held territory, has become a hot spot for concerns after tit-for-tat claims of attacks there.