He warned Russian President Vladimir Putin the international community "will be forced to apply costs" if Russia does not remove its troops from Crimea.
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Earlier, leaders of the G7 group of nations issued a similar threat.
 
Service etiquette 服務禮儀 Ukraine PM Arseniy Yatsenyuk, speaking after meeting Mr Obama, said Ukraine "will never surrender" to Russia.
 
Hotel design layout 酒店設計佈局 "It is absolutely unacceptable to have Russian boots on the Ukrainian ground in the 21st century, violating all international deals and treaties," he said.
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wuyuweiさんのブログ The diplomatic appeals to Moscow come ahead of Sunday's referendum in Crimea, in which citizens will be asked if they want to stay with Ukraine or join Russia.
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The Russian military and pro-Russian armed men moved in to seize key sites in Crimea - an autonomous region of Ukraine whose population is mainly ethnic Russian - in late February after the fall of President Viktor Yanukovych.