Russia's plans to deploy nuclear-capable missiles in Kaliningrad -- near Poland's border -- and its threat to move nuclear forces in Crimea would "fundamentally change the balance of security in Europe," Stoltenberg said in speech during a visit to Washington.
In blunt language, the NATO chief delivered a scathing critique of Russia's behavior over the past year -- including Moscow's armed intervention in Ukraine -- and vowed the transatlantic alliance would redouble its commitment to "collective defense."
Russian President Vladimir Putin's "admission that he considered putting Russia's nuclear forces on alert while Russia was annexing Crimea is but one example," said the former Norwegian prime minister.
The NATO alliance also was concerned about Russia's compliance with nuclear arms agreements and stepped up global flights by strategic bombers.
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"Russia has also significantly increased the scale, number and range of pro-active flights by nuclear capable bombers across much of the globe," Stoltenberg said.
Russia was failing to draw on the lessons of the Cold War, including that "when it comes to nuclear weapons, caution, predictability and transparency are vital," he said.
"Russia's nuclear sabre rattling is unjustified, destablizing and dangerous," he said.
Stoltenberg also criticized Russia for staging large snap military exercises, which he said was a violation of international agreements requiring governments to share information about planned drills and to invite observers.
One snap exercise was used to move Russian forces to annex Crimea in February 2014 and others were employed to support separatists in eastern Ukraine and to stage a military build-up on Ukraine's border, he said.