Crimea's Moscow-backed leaders declared a 96% vote in favour 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", US 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.

A pro-Russian protester celebrates in Simferopol's Lenin Square on March 16, 2014 after exit polls showed that about 95% of voters in Ukraine's Crimea region supported union with Russia (AFP photo)

But Obama said Russian forces must first end "incursions" into its ex-Soviet neighbour 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.
Moscow defended a military takeover of the majority ethnic Russian Crimea by citing a right to protect "peaceful citizens". Ukraine's interim government has mobilised troops to defend against an invasion of its eastern mainland, where pro-Russian protesters have been involved in deadly clashes in recent days.
With three-quarters of Sunday's votes counted in Crimea, a Black Sea peninsula that is home to 2 million people, 95.7% had supported annexation by Russia, chief electoral official Mikhail Malyshev, was quoted as saying by local media.
Turnout was 83 percent, he added - a high figure given that many who opposed the move had said they would boycott the vote. Russia's lower house of parliament will pass legislation allowing Crimea to join Russia "in the very near future", news agency Interfax cited its deputy speaker as saying on Monday.
"Results of the referendum in Crimea clearly showed that residents of Crimea see their future only as part of Russia," Sergei Neverov was quoted as saying. Japan on Monday echoed Western nations in rejecting the referendum and called on Russia not to annex Crimea. US and European officials say military action is unlikely over Crimea, which Soviet rulers handed to Ukraine 60 years ago.

But the risk of a wider Russian incursion, as Putin probes Western weakness and tries to restore Moscow's influence over its old Soviet empire, leaves NATO calculating how to help Kiev without triggering what some Ukrainians call "World War Three".
"We hope all parties can calmly maintain restraint to prevent the situation from further escalating and worsening.
Political resolution and dialogue is the only way out," Chinese Vice Foreign Minister Li Baodong told reporters on Monday, ahead of a visit to Europe by President Xi Jinping later this month. China avoided making a comment on the Crimea referendum and has said it does not back sanctions on Moscow - a close diplomatic ally and key economic partner.

Highlighting the stakes, journalist Dmitry Kiselyov, who is close to the Kremlin, stood before an image of a mushroom cloud on his weekly TV show to issue a stark warning. He said: "Russia is the only country in the world that is realistically capable of turning the United States into radioactive ash."
On Lenin Square in the centre of the Crimean capital Simferopol, a band struck up even before polls closed as the crowd waved Russian flags. Regional premier Sergei Aksyonov, a businessman nicknamed "Goblin" who took power when Russian forces moved in two weeks ago, thanked Moscow for its support.
The regional assembly is expected to rubber-stamp a plan to transfer allegiance to Russia on Monday before Aksyonov travels to Moscow, although the timing of any final annexation is in doubt. Putin may choose to hold off a formal move as diplomatic bargaining continues over economic and diplomatic sanctions that many EU states fear could hurt them as much as they do Russia.