The Russian president said earlier this week that the nuclear-armed country will “certainly use all means available to us” to defend itself if its territory is threatened.
In the wake of the Russian president’s threat, top EU diplomat, Josep Borrell, has warned that the war has reached a “dangerous moment” as he urged Western nations to be on high alert.
His warning comes as Moscow for the first time hinted at general mobilisation, allowing the Kremlin to call up all reservists and begin conscription.
"Certainly it’s a dangerous moment because the Russian army has been pushed into a corner, and Putin’s reaction - threatening using nuclear arms - it’s very bad," Mr Borrell told the BBC.
He said a “diplomatic solution” that “preserves the sovereignty and integrity of Ukraine” must be reached between both sides.
"Otherwise, we can finish the war, but we will not have peace, and we will have another war," he said.
In his address, Putin accused the West of engaging in “nuclear blackmail” and cited alleged “statements of some high-ranking representatives of the leading NATO states about the possibility of using nuclear weapons of mass destruction against Russia.”
He did not elaborate.
“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.
However, former Russian Prime Minister Mikhail Kasyanov has said the nuclear threat is nothing more than a desperate “bluff”.
Mr Kasyanov - who served as PM between 2000 and 2004 - believes his regime will collapse in the near future.
He said: “In terms of mobilisation, it’s not a bluff. In terms of potential use of nuclear weapons, I think it’s a bluff.
“He wants right now to black- mail the Western leaders, the whole of Western society.
“He wants to recognise occupied parts of Ukraine as Russian territory and to say to the Western leaders, ‘Putin is ready to use nuclear weapons to defend Russian territory’.
“But I very much doubt that Mr Putin will use nuclear weapons because he realises that for the Western leaders, that’s the same, whether there is annexation and recognition, the nature of the war would not be changed.”