The president said the “consequences” of any attempt to strike back following the country’s invasion of Ukraine would provoke a response “never seen in history”.
In an address to the UK, Prime Minister Boris Johnson said that “diplomatically, politically, economically – and eventually, militarily – this hideous and barbaric venture of Vladimir Putin must end in failure”.
Analysts believe the threat of deploying nuclear weapons should the Russian Army be attacked by forces outside of Ukraine was implicit in President Putin’s speech.
Broadcast live on television at 5.45am Moscow time, President Putin said: “Whoever tries to impede us, let alone create threats for our country and its people, must know that the Russian response will be immediate and lead to the consequences you have never seen in history.”
“All relevant decisions have been taken. I hope you hear me.”
Officials have said an attack on a military unit in Podilsk, outside Odessa in Ukraine, killed six people and wounded seven.
Russian tanks have have also crossed the Ukrainian border via Belarus.
NATO has now announced that it will take additional steps to strengthen the alliance’s deterrence and defence and officials said it would hold an emergency summit of its 30 member nations on Friday.
President Putin’s chilling address was a stark reminder to the West of Russia’s nuclear power.
“No one should have any doubts that a direct attack on our country will lead to the destruction and horrible consequences for any potential aggressor,” he said in his speech that acted as a declaration of war on Ukraine.
The invasion followed shortly after, leading to widespread horror and anguish as the human impact of the war began to take its toll.
Nuclear options represent the most drastic weapon in the armoury of superpowers, and the decision to use them carries the gravest of consequences.
While Russian nuclear weapons have the range to hit the UK, the possibility of nuclear weapons being used remains a long way off.
That’s because the devastation it could cause would outweigh any potential benefit from the mass destruction.
Britain’s nuclear deterrent is based in the Royal Navy and is in place to protect both the UK and its NATO allies.
Harrowing photos from Russia’s invasion have been shared across the world, with Ukrainian firefighters arriving to rescue civilians after an airstrike hit an apartment complex in Chuhuiv, Kharkiv Oblast.
A cyclist also appeared to have been hit by an explosive and knocked to the ground.
Elsewhere, camera footage posted by Ukrainian authorities has shown Russian military vehicles crossing into the country from Crimea.
A missile was filmed soaring across the sky before slamming into an airport in Ukraine as Russia’s air assault of the country intensifies.
And in Kyiv, large queues of traffic have been filmed as residents attempt to desperately flee the capital.
The road out of the city was blocked by heavy traffic, with countless families taking shelter in underground stations.
Russian forces later attempted to take Chernobyl power plant, leading to Ukraine’s president Volodymyr Zelenskyy describing Moscow’s action as a “declaration of war” against the whole of Europe.
He tweeted: “Russian occupation forces are trying to seize the #Chornobyl_NPP. Our defenders are giving their lives so that the tragedy of 1986 will not be repeated.
“Reported this to @SwedishPM. This is a declaration of war against the whole of Europe.”
The United Nations refugee agency said on Thursday that the situation in Ukraine was quickly deteriorating after Russia’s invasion and pleaded with neighbouring countries to keep their borders open to people seeking protection.
Meanwhile General Sir Richard Shirreff, ex-deputy supreme commander of Nato, warned the UK may be dragged into the conflict if Russia continues its march west.
He said: “There is a possibility that we as a nation will soon be at war with Russia. We in this country must recognise that our security starts not on the white cliffs of Dover – it starts in the forests of Latvia, Lithuania and Estonia.”