Russian President Vladimir Putin has warned Western countries against sending troops to Ukraine.
The consequences of such a decision would be "tragic", he said.
In his annual state of the nation address, President Putin accused the West of trying to drag Russia into an arms race.
At the same time, he said that Russia needed to strengthen its defences on its western border now that Sweden and Finland were joining Nato.
President Putin said the West "provoked" the conflict in Ukraine and "continues to lie, without any embarrassment, saying that Russia allegedly intends to attack Europe".
Probably referring to comments by French President Emmanuel Macron earlier this week that sending Nato ground troops to Ukraine "could not be excluded", President Putin said: "The consequences for possible interventionists will be... tragic."
"We also have weapons that can hit targets on their territory," he added.
"All this really threatens a conflict with the use of nuclear weapons and the destruction of civilisation. Don't they get that?"
Several Nato countries, including the US, Germany and the UK, ruled out deploying ground troops to Ukraine.
The US has criticised the Russian president's latest remarks regarding the possibility of nuclear warfare. State Department spokesman Matthew Miller said it was "not the first time we have seen irresponsible rhetoric from Vladimir Putin".
"It is no way for the leader of a nuclear armed state to speak," he said, adding that the US has seen no evidence that Moscow is planning to launch such weapons.
The speech lasted a record two hours and was attended by all senior politicians, the CEOs of oil and gas firms Rosneft and Gazprom, as well as religious leaders of all denominations.
It was broadcast on giant screens across Moscow, and several cinemas in Russian cities reportedly screened it free of charge.
As expected, there was no mention of the death of Alexei Navalny, the opposition leader who died in a Siberian penal colony two weeks ago and who many saw as President Putin's greatest opponent.
Navalny, who will be buried in Moscow on Friday, died under circumstances yet to be fully established on 16 February. His widow Yulia has insisted that President Putin was responsible.
President Putin also boasted about Russia's sophisticated weapons - like hypersonic aircraft and unmanned underwater vehicles - and said that Russia's strategic nuclear forces are in a "state of full readiness".
Notably, President Putin explicitly referred to the two years of fighting in Ukraine as a "war", despite his repeated insistence that Russia's invasion should be described as a "special military operation".
He said that an "absolute majority" of Russians supported his decision to invade Ukraine, and that the Russian people were now united against what he called Western attempts to weaken the country.
He also hit out at accusations from the US that Russia is developing nuclear weapons for use against satellites in space.
The speech came just over two weeks before Russia's presidential election, in which President Putin is widely expected to win a fifth term in office.
Kremlin spokesman Dmitry Peskov said the address could "to a large degree be seen as [Putin's] election programme".
Indeed, much of the address focused on domestic issues ranging from the modernisation of the tax system to pensions and incentives aimed at boosting Russia's dwindling birth rate.
He also talked about the need to improve the health of the nation, announcing a series of measures aimed at boosting Russia's life expectancy, which at 70 is currently one of the lowest in Europe.
Urging people to focus on physical activity and cut down alcohol consumption, he joked: "Stop drinking and start skiing!"
