President Vladimir Putin called up Russian military reservists on Wednesday, saying his promise to use all military means in Ukraine was "no bluff," and hinting that Moscow was prepared to use nuclear weapons.

His mobilisation call comes as Moscow-held regions of Ukraine prepare to hold annexation referendums this week, dramatically upping the stakes in the seven-month conflict by allowing Moscow to accuse Ukraine of attacking Russian territory.

A senior US official said Washington was taking Putin's "irresponsible" veiled threat to use nuclear weapons "seriously" and warned it could alter its "strategic posture" if need be.

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. Russia needed to support those in Ukraine who wanted to "determine their own future", he said.

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

- 'Act of desperation' -

"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.

"Those who are trying to blackmail us with nuclear weapons should know that the wind can also turn in their direction," Putin added.

But Ukrainian President Volodymyr Zelensky said in a interview with Germany's Bild media group released Wednesday, he did not think Putin would resort to nuclear weapons.

German Chancellor Olaf Scholz denounced the call-up as "an act of desperation" in a "criminal war" he said Russia could not win.

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". Shoigu said Moscow was "fighting not so much Ukraine as the collective West" in Ukraine.

In the wake of their announcements flights to neighbouring ex-Soviet countries were booked up for days to come, airline data showed, in what appeared to be a rush to quit the country. Prices for remaining seats skyrocketed.

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.

In a sweeping Ukrainian counter-offensive in recent weeks, Kyiv's forces have retaken hundreds of towns and villages that had been controlled by Russia for months.

In a rare admission of military losses from Moscow, Shoigu said on 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 were clearing rubble and broken glass from a nine-storey apartment block hit by an overnight missile strike in the eastern Ukrainian city of Kharkiv.

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

Her neighbour, 50-year-old Galina, expressed bewilderment.

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

The referendums follow a pattern 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 ballots, saying the international community would never recognise the results.

Beijing, which so far has tacitly backed Moscow's intervention called on Wednesday for a "ceasefire through dialogue" after Putin's address and in likely reference to the referenda said the "territorial integrity of all countries should be respected".

"It's irresponsible rhetoric for a nuclear power to talk that way. But it's not atypical for how he's been talking the last seven months and we take it very seriously," John Kirby, spokesman for the White House's National Security Council, told ABC's "Good Morning America".

Ukrainian President Volodymyr Zelensky told German media he does not believe Russia will use nuclear weapons warned against giving in to Putin.

"Tomorrow, Putin can say -- as well as Ukraine, we want part of Poland, otherwise we will use atomic weapons. We cannot make these compromises," he said.

- Strike at nuclear plant -

NATO Secretary General Jens Stoltenberg meanwhile denounced Putin's "dangerous and reckless nuclear rhetoric."

And EU foreign policy chief Josep Borrell on Wednesday accused Putin of putting world peace "in jeopardy".

"Putin's announcement of sham referenda, partial military mobilisation 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.

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.

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

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.