Russia's provocative rhetoric and its dramatic expansion of flights by nuclear bombers are deeply troubling and dangerous, NATO Secretary General Jens Stoltenberg said Wednesday.

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 warned, 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."

"Russia's recent use of nuclear rhetoric, exercises and operations are deeply troubling," he told an audience at the Center for Strategic and International Studies think tank.

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," the former Norwegian prime minister said.

The NATO alliance also was concerned about Russia's compliance with nuclear arms agreements and stepped up global flights by strategic bombers.

"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.

The Russian bomber flights, he said, spanned "from Japan to Gibraltar, from Crete to California, and from the Baltic sea to the Black Sea."

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 saber-rattling is unjustified, destabilizing and dangerous," he said.

- 'Aggressive nationalism' -

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 in advance and to invite observers.

One short-notice 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.

Russia is conducting yet another snap exercise near Ukraine this week that involves 250 aircraft and 700 pieces of heavy equipment, he said.

Dating back to Russia's intervention in Georgia in 2008, Moscow has sought to settle disputes through military force or the threat of military force, he said.

Russia has also started deploying its most modern weapons systems near the borders of NATO members in Eastern Europe, he said.

Citing Russia's actions in Ukraine and elsewhere, Stoltenberg said Moscow "is asserting its military power, stirring up aggressive nationalism, claiming the right to impose its will on its neighbors and grabbing land."

NATO's relations with Russia were at their lowest point in decades, he said.

"We are not back to the Cold War but we are far from a strategic partnership," he said.

The alliance needed to adapt to challenges, he said, "that may be with us for a long time."

NATO will uphold its principles, including respect for the sovereignty of states and transparency in military activities, while also renewing its commitment to "collective" defense and deterrence, he said.

Lithuania, Latvia and Estonia plan to make a formal request to NATO to deploy a force of several thousand troops in their countries as a counter-weight to Russia.

But it remains unclear if the alliance will approve the request.

The NATO chief's comments came a day after he held talks with President Barack Obama, who accused Russia of adopting an "increasingly aggressive posture."

Both Obama and Stoltenberg urged both sides in the Ukraine conflict to respect a shaky ceasefire accord.