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The United States accused Russia of plotting to annex more of Ukraine's territory via a "bogus" referendum as Moscow rejected a new peace initiative ahead of talks with the pan-European OSCE.
Western leaders including French President Francois Hollande warned that skirmishing in Ukraine's east could trigger all-out civil war, and see a May 25 presidential poll as crucial to hauling the country back from the brink.
But pro-Moscow rebels plan their own vote Sunday in the eastern cities of Donetsk and Luhansk, which US Secretary of State John Kerry said mirrored what happened with Ukraine's Crimea peninsula in March when it voted to join Russia.
"This is really the Crimea playbook all over again, and no civilised nation is going to recognise the results of such a bogus effort," Kerry told reporters.
"We flatly reject this illegal effort to further divide Ukraine.
"We are not going to sit idly by while Russian elements fan the flames of instability instead of fulfilling the commitments that we made," Kerry added, insisting that Putin was "calling all the shots".
Kiev and its Western backers believe Moscow is trying to ensure the referendum goes ahead in a bid to sow chaos ahead of the nationwide presidential elections two weeks later.
US Assistant Secretary of State for Europe Victoria Nuland said the May 25 election would be "the most pluralistic election that there has ever been in Ukraine".
"And far more pluralistic than anything that's been seen in Russia," she said.
Yesterday, the Organisation for Security and Cooperation in Europe (OSCE) urged a suspension of hostilities for the presidential election to take place.
"We need a ceasefire for the election," said OSCE chief Didier Burkhalter, who is also Swiss president and foreign minister. He is due today in Moscow to meet Putin.
Ukraine's Foreign Minister Andriy Deshchytsia called on 30 of his counterparts assembled at a Vienna meeting of the Council of Europe to help "eliminate the external threats and provocations supported by Russia".
But Russian Foreign Minister Sergei Lavrov said holding the vote during the current violence would be "unusual". Putin's spokesman has been more blunt, calling the idea "absurd".
