The United States warned Russia of punishing new sanctions if the Kremlin and its allies in Ukraine disrupt crucial presidential elections in the former Soviet republic less than 10 days away.
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".
"Far from defending the rights of the people in the east they are seeking to speak for everybody through the barrel of the gun," Kerry told reporters.
He said the United States and its European allies would impose sectorial sanctions "if Russia or its proxies disrupt the elections".
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.
Crucially however, the rebels were not invited despite Western calls for inclusive talks, and no progress was reported.
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.
