A Russian nuclear strike would change the course of the conflict and almost certainly trigger a "physical response" from Ukraine allies and potentially from NATO, a senior NATO official said on Wednesday.
Any use of nuclear weapons by Moscow would have "unprecedented consequences" for Russia, the official warned.
It would "almost certainly be drawing a physical response from many allies, and potentially from NATO itself", he said.
The official added that Moscow was using its nuclear threats mainly to deter NATO and other countries from directly entering its war on Ukraine.
The Kremlin on Wednesday scolded Western leaders for engaging in "provocative" nuclear rhetoric.
President Vladimir Putin on Sept. 21 warned the West he was not bluffing when he said he would be ready to use nuclear weapons to defend Russia against what he said was "nuclear blackmail" from major Western powers.
US President Joe Biden, who has cautioned that the world faced the biggest risk of nuclear Armageddon since the 1962 Cuban Missile Crisis, said he doubted that Putin would use a tactical nuclear weapon in Ukraine.
"We express our daily regret that Western heads of state, in the United States and Europe, engage in nuclear rhetoric every day," Kremlin spokesman Dmitry Peskov said of Biden's interview to CNN.
"We consider this a very harmful and provocative practice," Peskov said.
