The head of NATO says the alliance is worried Russia might extend its intervention beyond Crimea into eastern Ukraine and the crisis reflects a wider "strategy" by Moscow to exert control in the region.
"Our major concern right now is whether he (President Vladimir Putin) will go beyond Crimea, whether Russia will intervene in the eastern parts (of Ukraine)," Secretary General Anders Fogh Rasmussen said on Wednesday during an event in Washington.
Denouncing the moves in the Crimean peninsula as "military aggression", Rasmussen said Russia's actions were part of a long-running pattern across the region to block nations from forging ties with the West.
"If you look at all of this, you will see an overall Russian strategy," he told an audience at the Brookings Institution think tank.
"It serves their long-term strategic interests to keep instability in that region which can be used, among other things, to prevent countries in that region seeking Euro-Atlantic integration.
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"That's my main concern."
Rasmussen cited a "pattern" of "frozen, protracted conflicts" in breakaway regions in Georgia, Moldova, Azerbaijan and now Ukraine's Crimean peninsula.
Sending troops into Crimea was part of "a more long-term, Russian or at least Putin strategy", he added.
The NATO chief also said he expected alliance ministers to agree to bolstering assistance to Ukraine at an upcoming meeting but he did not specify what kind of aid might be approved.
US officials have said they are reviewing a request from Kiev for military support, including arms, ammunition and non-lethal equipment.
So far Washington has only agreed to provide military rations to Ukraine.
