A Russian nuclear strike against Ukraine would trigger "such a powerful answer" from the West that the Russian army would be "annihilated," said Josep Borrell, the EU's foreign policy chief.
His comments come in response to Russian President Vladimir Putin's increasingly combative rhetoric.
After signing a partial mobilisation decree to boost the Russian army ranks on 21 September, Putin warned NATO that his country had "various means of destruction" at its disposal to defend its "territorial integrity."
"It's not a bluff," Putin said.
In blunt remarks on Thursday, Borrell directly replied to the Russian leader's threats.
"There is the nuclear threat, and Putin is saying he is not bluffing. Well, he cannot afford bluffing," Borrell said during a European Diplomatic Academy event in Bruges.
"It has to be clear that the people supporting Ukraine and the European Union and the member states, and the United States and NATO are not bluffing neither."
"And any nuclear attack against Ukraine will create an answer -- not a nuclear answer but such a powerful answer from the military side -- that the Russian army will be annihilated, and Putin should not be bluffing," he said.
Borrell spoke of a "serious moment of history" and painted a grim picture of profound uncertainty and instability for global politics as a result of Russia's invasion.
The diplomat said the current rules-based system was being "challenged like never before".
"We are definitely out of the Cold War and the post-Cold War. The post-Cold War has ended with the Ukrainian war, with the Russian aggression against Ukraine," he told the audience.
"This war is changing a lot of things, and certainly it is changing the European Union. This war will create a different European Union, from different perspectives."
Borrell also described Europe as a "garden" of political freedom and economic prosperity but added the rest of the world was mostly a "jungle."
"The jungle could invade the garden. The gardeners should take care of it, but they will not protect the garden by building walls. A nice small garden surrounded by high walls in order to prevent the jungle from coming in is not going to be a solution. Because the jungle has a strong growth capacity, and the wall will never be high enough in order to protect the garden," he said.
"The gardeners have to go to the jungle. Europeans have to be much more engaged with the rest of the world."
Asked about Borrell's comments on nuclear annihilation, a European Commission spokesperson said a Russian nuclear strike against Ukraine would be a "total game-changer" and that EU countries were preparing for any possible scenario.
'I should be the best-informed guy in the world'
This is not the first time this week that Borrell delivers a surprisingly candid message.
On Monday, the diplomat declared the EU's era of dependencies, including on cheap Russian gas, to be over and said the bloc's soft power was dwindling.
"The United States took care of our security [...] China and Russia provided the basis of our prosperity. This is a world that is no longer there," he told the annual conference of EU ambassadors.
Borrell spoke of a world under a "messy multipolarity" structured around US-China competition which co-exists within a broader divide between democracies and authoritarian regimes.
"On our side, there are a lot of authoritarian regimes. We cannot say 'we are the democracies,' and the ones which follow us are also democracies -- that is not true," he said.
The EU's foreign policy chief confessed that European countries did not believe Russia was going to launch the full-scale invasion, despite repeated warnings from US officials.
"We did not believe that this was going to happen, and we did not foresee that Ukraine was ready to resist as fiercely and as successfully as they are doing," he said.
In what was arguably the most shocking moment of the speech, Borrell publicly berated the EU ambassadors present at the event for not reporting fast enough about developments in their respective countries.
"Sometimes, I knew more of what was happening somewhere by reading the newspapers than reading your reports. Your reports come sometimes too late," he told the ambassadors.
"Having all of you around the world, I should be the best informed person in the world -- at least as much as any Foreign Affairs Minister. I am 'Foreign Affairs Minister of Europe'. Behave as you would behave if you were an embassy."
Speaking at the same conference, European Commission President Ursula von der Leyen praised the ambassadors for doing "excellent work" on the ground.