President Barack Obama has threatened Vladimir Putin with sanctions after warning the Russian President that the United States has officially rejected the results of a secession referendum in Ukraine's Crimea region.
The White House said Obama made clear to Putin that the dispute could still be resolved diplomatically but that Russia first must halt military incursions into Ukrainian territory.
Regional authorities said late on Sunday night that, with more than half the vote counted, 95.5 percent had polled in favour of annexing the strategic peninsula by Moscow.
The regional government in Crimea, now controlled by Russian forces, went ahead with the ballot despite fierce opposition from the West.
"He (Obama) emphasised that Russia's actions were in violation of Ukraine's sovereignty and territorial integrity and that, in coordination with our European partners, we are prepared to impose additional costs on Russia for its actions," the White House said in a summary of Obama's telephone call with Putin.
Crimea referendum and independence
Crimea referendum and independence
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A man shows his shirt with the Russian emblem as he celebrates the results of the Crimean referendum at the Lenin Square in Simferopol
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An elderly retired Soviet Navy officer and his wife take a walk in Sevastopol the morning after the referendum
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A man plays accordion as people dance during celebrations in Sevastopol
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People wave Russian flags as fireworks explode in the sky over Sevastopol following the announcement of the result of the referendum
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A member of a Ukrainian "Maidan" self-defense battalion takes part in training to qualify for service in the newly-created National Guard.
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Pro-Russian protesters hold a Russian, Crimean and Soviet flags during their rally at Lenin Square in Simferopol, Ukraine
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A member of the Crimean election commission waits for voters at the polling station in Belogorsk near Simferopol, Crimea, Ukraine
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Polling stations opened in Crimea for a referendum about whether the Ukrainian Black Sea region should join Russia. The vote has been widely condemned by Western governments, who call it illegal and have announced sanctions against Russia if it goes ahead. Thousands of unmarked forces, believed to be Russian, have appeared in Crimea after local Moscow-backed authorities asked Russia for protection against 'extremists' in the new Ukrainian leadership
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A lettering on the facade of the Council of Ministers building reads 'Spring in Crimea' in Simferopol, Crimea, Ukraine
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People wave Crimean flags at Lenin square in Simferopol, Crimea, Ukraine
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A poster in Crimea presents a stark choice - Nazism, or Russia - to voters ahead of the referendum
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Protesters against Ukraine’s referendum gather in Simferopol
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Action stations: Preparations for today’s referendum in Simferopol, where Crimea will vote to become part of Russia
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Cossacks guard the regional parliament building in Simferopol during the Crimean referendum
Obama suggested to Putin that the referendum was a sham carried out "under duress of Russian military intervention" and Washington and the international community would never accept the results, the White House said.
According to the Kremlin, Putin told Obama the referendum on union with Russia was legitimate the two leaders agreed on the need to cooperate to stabilise Ukraine.
Some officials in Washington are said to be hopeful that instead of pressing ahead with such a provocative action, Putin may hold off for now, seeking to avoid further escalation of the crisis.
White House senior adviser Dan Pfeiffer said the administration would step up pressure on Russia. "You can expect sanctions designations in the coming days," Pfeiffer told NBC's "Meet the Press," as the administration prepared to identify Russians whom the United States will seek to punish with visa bans and asset freezes the president authorized earlier this month.
While Washington and its allies have ruled out military action, a sanctions announcement is expected on Monday. Foreign ministers from the European Union, which has major trade ties with Russia, will decide on possible similar action in Brussels on the same day, Western sources said.
At the same time, the Obama administration is mindful that Russia could retaliate with steps of its own. Any efforts to punish Moscow are complicated by the need for cooperation on Iran nuclear diplomacy, removal of Syrian chemical weapons and use of Russian territory for U.S. withdrawal from Afghanistan.
Republican Senator John McCain of Arizona, just back from a visit to Ukraine, urged the administration to provide military assistance to Ukraine, resume development of a US missile defense system for Eastern Europe and take steps toward NATO membership for Georgia and Moldova.
"The United States of America has to first of all have a fundamental reassessment of our relationship with Vladimir Putin. No more 'reset' button," McCain told CNN, referring to Obama's outreach to Russia early in his first term.
In a phone conversation with Russian Foreign Minister Sergei Lavrov, U.S. Secretary of State John Kerry said Russian forces must return to their bases and objected to Moscow's military activities in Ukraine's Kherson oblast area and continuing "provocations" in Eastern Ukraine, a senior State Department official said.
Lavrov and Kerry, according to a Russian statement, "agreed to continue work to find a resolution on Ukraine through a speedy launch of constitutional reform."