President Vladimir Putin warned of a global nuclear conflict if Nato allies sent troops to support Ukraine as he claimed the West was trying to 'destroy' Russia.
Mr Putin spoke during a State of the Nation address on Thursday, two weeks ahead of a presidential election in which he is expected to win another six-year term.
French President Emmanuel Macron this week said that he had 'not ruled out' putting boots on the ground in Ukraine – a suggestion that was quickly rejected by the US, Germany, Britain and others.
Mr Putin said the consequences of such a military intervention would be “tragic” and that his nuclear forces were in a “state of full readiness for guaranteed use”.
“We also have weapons that can hit targets on their territory. All this really threatens a conflict with the use of nuclear weapons and the destruction of civilisation. Don't they get that?”
The speech comes as leaked documents published in the Financial Times revealed Russia's threshold for tactical nuclear strikes was lower than had been publicly admitted previously.
Criteria for a nuclear strike consist of an incursion that threatened the Russian state, or the destruction of 20 per cent of Russia's strategic ballistic submarines, according to the report.
The leaked cache consists of 29 secret Russian military files drawn up between 2008 and 2014 that are still in use by the Kremlin's military hierarchy. They also provide training scenarios for an invasion by China.
Tensions were further increased this week after German Chancellor Olaf Scholz revealed that the British Army was helping Kyiv ground forces to use one of its missile systems on the ground in Ukraine.
Mr Scholz said that by sending Taurus cruise missiles to Ukraine it would make Germany a 'participant in the war'.
Politicians in Britain branded the comments 'dangerous' and 'irresponsible' and gave Mr Putin a pretext to escalate his nuclear rhetoric.
Mr Putin added that Russian forces now had the initiative on the battlefield in Ukraine and were advancing in several places.
Russia must also increase the troops it has deployed along its western borders with the EU after Finland and Sweden decided to join the Nato military alliance, he added.
The veteran leader dismissed western suggestions that Russian forces might go beyond Ukraine and attack European countries as “nonsense”.
He also said Moscow would not repeat the mistake of the Soviet Union and allow the West to “drag” it into an arms race that would eat up too much of its budget.
“Therefore, our task is to develop the defence-industrial complex in such a way as to increase the scientific, technological and industrial potential of the country,” he said.
Mr Putin said Moscow was open to discussions on nuclear strategic stability with the US but suggested that Washington had no genuine interest in such talks.
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“Recently there have been more and more unsubstantiated accusations against Russia, for example that we are allegedly going to deploy nuclear weapons in space. Such innuendo … is a ploy to draw us into negotiations on their terms, which are favourable only to the United States,” he said.
“On the eve of the US presidential election, they simply want to show their citizens and everyone else that they still rule the world.”
Mr Putin is expected to win by a landslide when the Russian presidential election takes place between March 15 and 16. He faces opposition from three other candidates nominated by Kremlin-friendly parties represented in parliament.
Russia’s best-known opposition leader Alexei Navalny, whose attempt to run against Mr Putin in 2018 was rejected, died suddenly in an Arctic prison colony earlier this month while serving a 19-year sentence on extremism charges. Mr Navalny’s funeral is scheduled for Friday.
