The first meeting between Obama and Ukrainian Prime Minister Arseniy Yatsenyuk comes with Europe's largest nation in danger of breaking apart when the predominantly ethnic Russian region holds a Moscow-backed referendum Sunday on switching over to Kremlin rule.
Ukraine's acting President Oleksandr Turchynov told AFP his heavily outnumbered army would never try to seize back the Black Sea peninsula from Russian troops who made their land grab days after the February 22 ouster in Kiev of pro-Kremlin leader Viktor Yanukovych.
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"We cannot launch a military operation in Crimea, as we would expose the eastern border and Ukraine would not be protected," Turchynov said in an interview with AFP.
Turchynov also said Russian President Vladimir Putin has so far resisted intense international pressure and refused all contacts with Kiev aimed at resolving the worst breakdown in East-West relations since the Cold War.
"Unfortunately, for now Russia is rejecting a diplomatic solution to the conflict," he said. "They are refusing all contact at foreign ministry and top government level."
Russia's first military involvement in a neighbouring country since its brief 2008 war with Georgia has sparked an explosive security crisis and exposed major rifts between Western allies over ways to deal with Putin's undisguised efforts to rebuild vestiges of the Soviet state.
Washington has imposed travel bans and asset freezes on Russians held responsible for violating the territorial integrity of the culturally splintered nation of 46 million people.
But the European Union -- its financial and energy sectors much more dependent on Russia than those of the United States -- has only threatened tougher measures after taking the lighter step of suspending free travel and broad economic treaty talks.