Slavyansk - The United States accused Russia of plotting to annex more of Ukraine's territory via a "bogus" referendum as President Vladimir Putin 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 25 May presidential poll as crucial to hauling the country back from the brink.
But pro-Moscow rebels plan their own vote on 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 25 May 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.
On Tuesday, the Organisation for Security and Co-operation 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 Wednesday 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 spokesperson has been more blunt, calling the idea "absurd".
Lavrov also dismissed a European push spearheaded by Germany to hold fresh peace talks on Ukraine after the collapse of a 17 April agreement that Moscow has already declared dead.
Ukraine has long been riven by political crisis over whether to pursue a pro-Russia course or align more closely with the European Union, and the bloodshed from fighting on the ground is now mounting.
Nearly 90 people have died in less than a week: half around the eastern town of Slavyansk, held by rebels since early April; and half in the southern port city of Odessa, where clashes culminated in a deadly inferno on Friday.
Most of those killed have been pro-Russia fighters and activists.
Putin has so far not ordered a threatened operation to "protect" Ukraine's Russian-speaking population, but he has kept an estimated 40 000 troops on the border for the past two months.
The US general commanding Nato's military operations, Philip Breedlove, said Russian special forces the West alleges are deployed covertly "may be able to accomplish his [Putin's] objectives in eastern Ukraine" without the need to invade.
But speaking after meeting Kerry in Washington, EU foreign policy chief Catherine Ashton renewed calls for Moscow to help end "illegal actions by armed separatist groups".
"Any further steps that destabilize the situation in Ukraine would lead to additional and far-reaching consequences for our relations in a broad range of areas," she warned.
US sanctions against Moscow over the escalating unrest have taken a toll on the Russian economy, Daniel Glaser, the US Treasury assistant secretary for terrorist financing, told a hearing in Congress.
The limited sanctions so far have stimulated a heavy capital flight from Russia, hobbled a bank close to Putin and taken economic growth to near zero.
If Moscow does not stop interfering in Ukraine and supporting pro-Russia separatists, the US will implement more sanctions, Glaser warned.
As the two votes near, Ukraine's authorities are stepping up their offensive to crush rebels holed up in Slavyansk, the epicentre of the insurgency.
Four members of the Ukrainian security forces were killed and 20 wounded there in fierce fighting on Monday, Interior Minister Arsen Avakov said.
"According to our estimates over 30 terrorists were killed and dozens were wounded," he added.
An AFP reporter inside Slavyansk said basic foodstuffs and other items were running increasingly short.