"This is a wake-up call," Secretary-General Anders Fogh Rasmussen yesterday 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.
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.
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He demanded Moscow halt all military activities against Ukraine and seek a peaceful dialogue with Ukraine's government.
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.
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.