Russian President Vladimir Putin announced a partial mobilisation in Russia as the war in Ukraine reaches nearly seven months and Moscow loses ground on the battlefield.
He also warned the West that "it's not a bluff" that Russia would use all the means at its disposal to protect its territory.
The total number of reservists drafted in the partial mobilisation is 300,000, officials said.
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Putin's address to the nation comes a day after Russian-controlled regions in eastern and southern Ukraine announced plans to hold votes on becoming integral parts of Russia.
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.
Putin accused the West of engaging in "nuclear blackmail" and noted "statements of some high-ranking representatives of the leading NATO states 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 and 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.
He added: "It's not a bluff".
Putin said he has signed a decree on the partial mobilisation, which is due to start on Wednesday.
"We are talking about partial mobilisation, 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.
Russian Defence Minister Sergei Shoigu said in a televised interview on Wednesday that Shoigu assured that conscripts and students will not be mobilised, only those with relevant combat and service experience will.
He said that 5937 Russian troops have been killed in Ukraine so far. Western estimates of Russian military losses stand at tens of thousands.
Shoigu's update on Russian losses is the third time the Russian military offered death toll numbers to the public. The last update came in late March, when the Defence Ministry claimed 1351 Russian soldiers were killed in Ukraine.
The referendums, which have been expected to take since the first months of the war, will start Friday in the Luhansk, Kherson and partly Russian-controlled Zaporizhzhia and Donetsk regions.
Putin said the decision to partially mobilise was "fully adequate to the threats we face, namely to protect our homeland, its sovereignty and territorial integrity, to ensure the security of our people and people in the liberated territories."
On Tuesday, Ukrainian President Volodymyr Zelenskyy dismissed Russian plans to stage the referendums as "noise" and thanked Ukraine's allies for condemning the votes.
Former President Dmitry Medvedev, the deputy head of Russia's Security Council chaired by Putin, said referendums that fold regions into Russia itself would make redrawn frontiers "irreversible" and enable Moscow to use "any means" to defend them.
In his nightly address, Zelenskyy said there were lots of questions surrounding the announcements but stressed that they would not change Ukraine's commitment to retake areas occupied by Russian forces.
"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."
The upcoming votes, in the Luhansk, Kherson, Zaporizhzhia and Donetsk regions are all but certain to go Moscow's way.
But they were quickly dismissed as illegitimate by Western leaders who are backing Kyiv with military and other support that has helped its forces seize momentum on battlefields in the east and south.
"I thank all friends and partners of Ukraine for today's mass principled firm condemnation of Russia's attempts to stage new sham referenda," Zelenskyy said.
In another signal that Russia is digging in for a protracted and possibly ramped-up conflict, the Kremlin-controlled lower house of parliament voted Tuesday to toughen laws against desertion, surrender and looting by Russian troops. Lawmakers also voted to introduce possible 10-year prison terms for soldiers refusing to fight.
If approved, as expected, by the upper house and then signed by Putin, the legislation would strengthen commanders' hands against failing morale reported among soldiers.
In the Russian-occupied city of Enerhodar, shelling continued around Europe's largest nuclear power plant.
Ukrainian energy operator Energoatom said Russian shelling again damaged infrastructure at the Zaporizhzhia nuclear power plant and briefly forced workers to start two diesel generators for emergency power to the cooling pumps for one of the reactors.
Such pumps are essential for avoiding a meltdown at a nuclear facility even though all six of the plant's reactors have been shut down. Energoatom said the generators were later switched off as main power was restored.
The Zaporizhzhia Nuclear Power Plant has been a focus of concern for months because of fears that shelling could lead to a radiation leak. Russia and Ukraine blame each other for the shelling.
