As temperatures soar, NATO Secretary-General Jens Stoltenberg says Russia's Vladimir Putin would be crossing a “very important line” if he were to order the use of nuclear weapons.
EU foreign policy chief Josep Borrell has warned Moscow that its forces would be "annihilated" by the West's military response if President Vladimir Putin uses nuclear weapons against Ukraine.
Thursday's remark came amid concerns about the possibility of Moscow using tactical nuclear weapons after suffering setbacks in Ukraine.
Fears have grown after Putin issued veiled threats as he staged the annexation of four occupied regions in the face of losses on the battlefield.
"Putin is saying he is not bluffing. Well, he cannot afford bluffing, and 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," Borrell said at the opening of a Diplomatic Academy in Belgium.
The US and NATO has so far steered clear of intervening militarily in the Ukraine conflict for fear of sparking a catastrophic nuclear conflict with Moscow.
"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," Borrell added.
'Very important line'
Separately, NATO Secretary-General Jens Stoltenberg said Putin would be crossing a "very important line" if he were to order the use of nuclear weapons in Ukraine.
Asked what NATO would do if Russia launched a nuclear attack, Stoltenberg said: "We will not go into exactly how we will respond, but of course this will fundamentally change the nature of the conflict. It will mean that a very important line has been crossed."
He added that "even any use of a smaller nuclear weapon will be a very serious thing, fundamentally changing the nature of the war in Ukraine, and of course that would have consequences."
The remarks came after a meeting of NATO's secretive Nuclear Planning Group, which was held among defence ministers in Brussels.
The 30-member military alliance has stopped short of threatening to use its nuclear arsenal to respond as non-member Ukraine is not covered by its mutual self-defence clause.
With temperatures soaring, observers have pointed out that Putin has repeatedly alluded to using his country's vast nuclear arsenal.
"I want to remind you that our country also has various means of destruction... and 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 on September 21, adding with a lingering stare at the camera, "It’s not a bluff."