"This is a wake-up call," Secretary-General Anders Fogh Rasmussen said in a speech in Washington. "For the Euro-Atlantic community. For NATO. And for all those committed to a Europe whole, free and at peace."

There had been crises in the Balkans in the 1990s and in Georgia in 2008, "but this is the gravest threat to European security and stability since the end of the Cold War," he said.

The situation was more serious because of the scale of Russia's action, representing one of the largest troop movements in Europe for "many decades," and because it was occurring "right on NATO's border," he said.

Rasmussen, who was holding talks with US leaders on the crisis, denounced Russia's actions as "military aggression" that violated Ukraine's sovereignty.

Russia's "attempt to justify the annexation of Crimea through a so-called referendum held at gunpoint is illegal and illegitimate," he told an audience at the Brookings Institution think tank.

He demanded Moscow halt all military activities against Ukraine and seek a peaceful dialogue with Ukraine's government.

But if it keeps "on its current course, Russia is choosing increased international isolation," said Rasmussen, in a reference to threatened economic sanctions against Moscow.

But he issued no military threat and acknowledged the West had no "easy" options at hand.

"There are no quick and easy ways to stand up to global bullies.

"Because our democracies debate, deliberate and consider the options before taking decisions. Because we value transparency and seek legitimacy for our choices.

"And because we see force as the last, not the first, resort."

Rasmussen spoke as Pentagon officials said the US military was looking at expanding military cooperation with NATO allies on Russia's border, beyond already scheduled drills in April and June in Baltic countries.

He said Russia had pledged to respect Ukraine's territorial integrity in 1994 but "turned its back" on the agreement with its move into the Crimean peninsula.

NATO had suspended staff-level military and civilian meetings with Russia and was reviewing all cooperation with Moscow in light of the crisis, he said.

It was no longer clear if Russia was a partner or an "adversary," Rasmussen said, adding that future NATO deliberations needed "to focus on the long-term strategic impact of Russia's aggression on our own security."

The NATO chief's visit to Washington coincided with soaring tensions in Ukraine, with the acting president warning Crimea's pro-Kremlin leaders they had only three hours to release the captured head of the country's navy or face "an adequate response."

Russia has tightened its hold on the Crimean peninsula despite warnings and condemnation from the West.