Kiev's interim leaders are battling to keep Ukraine from disintegrating ahead of the May 25 vote, pressing on with a military offensive to put down a bloody pro-Moscow insurgency in the eastern industrial heartland.
"Our message is really, quite simple: 'Let Ukraine vote. Let the Ukrainian people choose their future'," US Secretary of State John Kerry said at a meeting with European counterparts in London.
He said the separatists who have seized over a dozen towns in a month of fighting and declared independence in two industrial regions in defiance of Kiev and the West were "sowing mayhem".
He said the United States and its European allies would impose sectorial sanctions "if Russia or its proxies disrupt the elections".
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The tough talk came as Ukraine pressed on with a military operation against rebels around Slavyansk, the epicentre of the uprising, despite the launch of "national unity" talks in Kiev on Wednesday.
The round-table discussions are part of a initiative launched by the pan-European OSCE to try to resolve the escalating crisis on Europe's doorstep.
The east of Ukraine remains on edge, with fighting flaring almost every night around rebel flashpoints and reports -- difficult to confirm -- of attacks on election centres and government officials.