With Russian troops reportedly on the move in Crimea late Friday, President Barack Obama warned "there will be costs" for direct intervention, saying Russian military action "would represent a profound interference in matters that are the business of (the) Ukrainian people."
But Ukraine and the West have few realistic options to put into play against Russia.
"If Crimea decides it wants to be part of Russia, all Ukraine can hope to do is get the EU and U.S. to intervene on its behalf," said Olga Oliker, associate director of Rand Corp.'s International Security and Defense Policy Center.
But direct intervention by Western powers is highly unlikely, Oliker added.
Hard-line members of the Russian-speaking majority on the Crimean peninsula called for the region to secede from Ukraine this week, and political leaders in the region have vocally opposed new leadership in the capital Kiev.
Russian armed forces announced unexpected military exercises in the area, and on Thursday, gunmen bearing the Russian flag seized government buildings in Crimea. On Friday, armed men took control of two Crimean airports, bringing Ukrainian accusations of a Russian invasion directed by President Vladimir Putin.
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"It is very clear that Russia is positioning itself to formulate a secessionist movement there," said Ian Brzezinski, a senior fellow at the Atlantic Council focused on trans-Atlantic security.
"These guys in uniforms, all carrying the same assault weapons—these are not indigenous groups out of Crimea," Brzezinski said of the gunmen who occupied government buildings in Crimea before Russian troops began to move. "This is an organized, orchestrated effort by clearly trained military units. These forces didn't show up without Putin's approval."