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
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