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By Mike Collett-White and Alastair Macdonald SIMFEROPOL/KIEV (Reuters) - Crimea's Moscow-backed leaders declared a 96-percent vote in favor of quitting Ukraine and annexation by Russia in a referendum Western powers said was illegal and will bring immediate sanctions. As state media in Russia carried a startling reminder of its power to turn the United States to "radioactive ash", President Barack Obama spoke to Vladimir Putin, telling the Russian president that he and his European allies were ready to impose "additional costs" on Moscow for violating Ukraine's territory. The Kremlin and the White House issued statements saying Obama and Putin saw diplomatic options to resolve what is the gravest crisis in East-West relations since the Cold War. But Obama said Russian forces must first end "incursions" into its ex-Soviet neighbor while Putin renewed his accusation that the new leadership in Kiev, brought to power by an uprising last month against his elected Ukrainian ally, were failing to protect Russian-speakers from violent Ukrainian nationalists.