WASHINGTON After two decades of trying to build a partnership with Russia, NATO now feels compelled to start treating Moscow as an adversary, the alliance's second-ranking official said Thursday.

"Clearly the Russians have declared NATO as an adversary, so we have to begin to view Russia no longer as a partner but as more of an adversary than a partner," said Alexander Vershbow, the deputy secretary-general of NATO.

In a question-and-answer session with a small group of reporters, Vershbow said Russia's annexation of Crimea and its apparent manipulation of unrest in eastern Ukraine have fundamentally changed the NATO-Russia relationship.

"In central Europe, clearly we have two different visions of what European security should be like," Vershbow, a former U.S. diplomat and onetime Pentagon official, said. "We still would defend the sovereignty and freedom of choice of Russia's neighbors, and Russia clearly is trying to re-impose hegemony and limit their sovereignty under the guise of a defense of the Russian world."

In April, NATO suspended all "practical civilian and military cooperation" with Russia, although Russia has maintained its diplomatic mission to NATO, which was established in 1998.

The crisis over Ukraine and its implications for U.S. and NATO relations with Russia has become a major point of concern for top Obama administration officials, including Defense Secretary Chuck Hagel, who spoke by phone Monday to Russian Defense Minister Sergei Shoigu about Russia's intentions in Ukraine. Aides said Hagel was assured that Russia has no intention of invading.

Hagel is scheduled to speak Friday at the Woodrow Wilson International Center for Scholars on European security in the context of the Ukraine crisis and the deepening chill in relations with Moscow.

Vershbow said NATO, created 65 years ago as a bulwark against the former Soviet Union, is considering new defensive measures aimed at deterring Russia from any aggression against NATO members along its border, such as the Baltic states that were once part of the Soviet Union, Vershbow said.

"We want to be sure that we can come to the aid of these countries if there were any, even indirect, threat very quickly before any facts on the ground can be established," he said.

To do that, NATO members will have to shorten the response time of its forces, he said.

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NATO official says Russia must now be viewed as more of an enemy than a partner