Russia is trying to test NATO's mettle and members of the transatlantic alliance must bolster military spending in the face of Moscow's challenge, US Defense Secretary Chuck Hagel warned Friday.

The 28 members of NATO have responded to Russia's intervention in Ukraine with "resolve," Hagel said, "but over the long term, we should expect Russia to test our alliance's purpose, stamina and commitment."

Russia's annexation of Crimea and its backing of separatists in eastern Ukraine "has reminded NATO of its founding purpose," he said.

After the Cold War, the alliance had looked to forge a partnership with its old adversary.

But after Moscow's moves in Ukraine, "NATO must stand ready to revisit the basic principles underlying its relationship with Russia," he said.

Hagel's press secretary, Rear Admiral John Kirby, declined to describe Russia as an "enemy" of the United States.

NATO's deputy secretary general, US diplomat Alexander Vershbow, however said Thursday that Moscow had be to seen as "more of an adversary than a partner."

- Crucial choice -

In his speech, Hagel told an audience at the Wilson Center think tank in Washington that the alliance faced a crucial choice in light of Russia's assertive stance in Ukraine -- to stand firm or to retreat.

"Future generations will note whether... at this moment of challenge, we summoned the will to invest in our alliance," he warned.

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NATO must invest in defense to counter Russia: US