Stoltenberg stressed however that there would be "serious consequences" for Russia if it uses nuclear weapons in Ukraine.
The circumstances in which NATO allies might have to resort to nuclear weapons are "extremely remote" but Russia would face "severe consequences" if it uses such means against Ukraine, the alliance's Secretary-General stressed on Thursday.
The comments from Jens Stoltenberg were made at the end of a two-day meeting of NATO defence ministers in Brussels.
"Circumstances in which NATO might have to use nuclear weapons are extremely remote. Russia's nuclear rhetoric is dangerous, reckless, and they know that if they use nuclear weapons against Ukraine, it will have severe consequences. And they also know that a nuclear war cannot be won and should never be fought," he told reporters.
Several allies used the ministerial meeting to increase their contribution to Ukraine's war effort including with much-needed air defence systems.
Moscow targeted Ukrainian cities with a wave of missile strikes earlier this week in retaliation for the explosion on Saturday of the bridge linking Russia to the Crimean peninsula, illegally annexed in 2014.
Faced with a decisive counter-offensive from Ukrainian forces that has led to the recapture of large swathes of territories in the east and in the south of the country over the past few weeks, Russia has also rapidly announced the illegal annexation of four areas following hastily-organised sham referendums.
Its president, Vladimir Putin, has also made veiled references to the potential use of nuclear weapons, stating that Russia would protect its territory through "all means available".
According to the NATO chief, Russia does not seem to be readying its nuclear arsenal for now.
US Defence Secretary Lloyd J. Austin said on Thursday at the NATO summit that allies continue "to make clear that we will not be dragged into Russia's war of choice. But we will stand by Ukraine as it fights to defend itself."
Meanwhile, 15 European allies have agreed to move ahead with the creation of a European Sky Shield Initiative designed to plug existing gaps in the continent's air defence capabilities.