File poto showing NATO Secretary General Jens Stoltenberg talks during a news conference at the NATO summit in Madrid, Spain, on Wednesday, June 29, 2022. AP
"This is dangerous and reckless nuclear rhetoric. It's not new as he has done it many times before. He knows very well that a nuclear war should never be fought and cannot be won, and it will have unprecedented consequences for Russia," Stoltenberg said on the sidelines of the United Nations General Assembly in New York.
Stoltenberg said NATO would "make sure that there is no misunderstanding in Moscow" about the alliance's willingness to defend itself.
"Exactly how we will react, of course, depends on what kind of situation and what kind of weapons they may use," he said at an event hosted by Reuters news agency.
"The most important thing is to prevent that from happening."
The NATO leader said that so far the Western military alliance had not seen any changes in Russia's nuclear posture, "but we monitor this very closely and we stay vigilant".
Putin said in a speech on Wednesday that Moscow would use "all the means at our disposal to protect Russia and our people" and ordered a partial military mobilisation after a series of setbacks in Ukraine.
"The speech of President Putin demonstrates that the war is not going according to plan, he has made a big miscalculation," Stoltenberg said.
He said the mobilisation "will escalate the conflict and that will mean more suffering, more loss of lives".
But he said Moscow could struggle to deploy any new forces effectively after problems so far with equipment and command and control over troops.