Russian President Vladimir Putin has ordered a partial military mobilisation in Russia, with 300,000 reservists set to be called up as the Kremlin attempts to regain ground in the face of a counter-attack by Ukraine’s forces.
It is Russia's first mobilisation since the Second World War and comes amid battlefield losses for Russian forces in recent weeks.
In a televised address, Mr Putin warned the West that Russia would use all the means at its disposal to protect itself, which is being viewed as a reference to Russia's nuclear capability.
"It's not a bluff," Mr Putin said.
He also 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."
The Defence Secretary said Mr Putin's partial mobilisation announcement is "an admission that his invasion is failing".
"President Putin’s breaking of his own promises not to mobilise parts of his population and the illegal annexation of parts of Ukraine, are an admission that his invasion is failing," Mr Wallace said.
"He and his Defence Minister have sent tens of thousands of their own citizens to their deaths, ill equipped and badly led. 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."
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Foreign Office minister Gillian Keegan questioned whether Mr Putin was "in control".
She told Sky News: "Some of the language there was quite concerning at the end and obviously we would urge for calm."
The Chichester MP also said: "It's something that we should take very seriously because, you know, we're not in control.
"I’m not sure he's in control either, really. I mean, this is obviously an escalation and, of course, for the Russian people now they will be conscripted into this war."
Melinda Simmons, the UK’s ambassador in Kyiv, said Mr Putin's "essential weakness" was "he still refuses to understand Ukraine".
Kremlin spokesman Dmitry Peskov, who was asked what had changed since he and others previously said no mobilisation was planned, argued that Russia was effectively fighting against a combined potential of NATO because the alliance’s members had been supplying weapons to Kyiv.
Only those with relevant combat and service experience would be mobilised, Russian defence minister Sergei Shoigu said.
He added that there were around 25 million people who fitted this criteria, but only around 1% of them would be mobilised.
Another clause in the decree prevents most professional soldiers from terminating their contracts and leaving service until the partial mobilisation is no longer in place.
A British defence intelligence update suggested Mr Putin was being forced to undermine his own public position that the war in Ukraine was a "special military operation" rather than a full-scale conflict.
"These new measures have highly likely been brought forwards due to public criticism and mark a further development in Russia's strategy," the Ministry of Defence said.
"Putin is accepting greater political risk by undermining the fiction that Russia is neither in a war nor a national crisis in the hope of generating more combat power."
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A former adviser to Mr Putin suggested the Russian leader would be ready to use nuclear weapons against Western nations such as the UK.
Political scientist Sergei Markov said it "could kill a lot of people in the Western countries".
Mr Markov added: "This nuclear war could be a result of the crazy behaviour of the president of the United States Joe Biden and prime ministers of Great Britain Boris Johnson and Liz Truss."
Mr 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 recent Ukrainian successes on the battlefield.
The referendums will start on Friday in the Luhansk, Kherson and partly Russian-controlled Zaporizhzhia and Donetsk areas.