In the biggest escalation of the Ukraine war since Moscow's Feb. 24 invasion, Putin explicitly raised the specter of a nuclear conflict.

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".

"If the territorial integrity of our country is threatened, we will without doubt use all available means to protect Russia and our people - this is not a bluff," Putin said in a televised address to the nation.

"In its aggressive anti-Russian policy, the West has crossed every line," Putin said. "This is not a bluff. And those who try to blackmail us with nuclear weapons should know that the weathervane can turn and point towards them."

The Russian president's decision to order a partial mobilization was because of the low morale among his forces, said Ukraine President Vlodomyr Zelenskyy.

"He needs an army of millions... he sees that a large part of those (troops) who come to us, just run away," he said.

Putin "wants to drown Ukraine in blood, also the blood of his own soldiers", Zelensky said.

The Putin address, which followed a critical Russian battlefield defeat in northeastern Ukraine, fueled speculation about the course of the war, the 69-year-old Kremlin chief's own future, and showed Putin was doubling down on what he calls his "special military operation" in Ukraine.

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.

The callup of more soldiers comes as Moscow has its regimes in the "breakaway" Donetsk and Luhansk "Peoples Republics" staging "referendums" to join Russia denounced by the West as puppet actions.

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.

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 recognize the results.

German Chancellor Olaf Scholz said "Sham referendums" in four Russian-occupied regions of Ukraine would "of course never be accepted" by the international community, Scholz said, and would hence be "no justification" for Russia's "intention, namely to conquer land of its neighbor with violence".

"In the world in which we live, the law must win out over force and force can never be stronger than the law," he said.

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.

"This is just another proof of Putin that he is not interested in peace, that he's interested in escalating his war of aggression," said EU spokesman Peter Stano.

"This is also yet another sign of his desperation with how his aggression is going against Ukraine."

EU foreign policy chief Josep Borrell on Wednesday accused Russian leader Vladimir Putin of putting world peace "in jeopardy" by threatening to use nuclear weapons over the war in Ukraine.

"Putin's announcement of sham referenda, partial military mobilization and nuclear blackmail are a grave escalation," Borrell wrote on Twitter.

"Threatening with nuclear weapons is unacceptable and a real danger to all," he said.

"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 Zelenskyy said in response.

The Ukrainian president has made it clear that his forces will work to retake all territory taken by Russia since 2014. And in recent weeks, Ukraine's military has indeed made striking advances.

This report was compiled using information from Reuters and Agence France-Presse