The Central Election Commission called for the authorities in Kiev to take urgent action to ensure security in the east, saying almost two million people could be prevented from voting on May 25 because of the rebellion.
Its warning came as Ukraine's embattled Western-backed government was preparing to hold a second round of "national unity" dialogue called for under an OSCE-sponsored peace plan.
Although the talks are taking place later today in the eastern city of Kharkiv - which has been rattled by violence since the rebellion erupted - no separatist leaders have been invited to the table to Russia's annoyance.
The West see the election as crucial to defusing the crisis on Europe's eastern flank and preventing the former Soviet republic from disintegrating further after Russia's annexation of Crimea.
In total, 36 million Ukrainians are eligible to vote on May 25 in an election expected to deliver victory to billionaire chocolate baron Petro Poroshenko.
Pro-Russians in the east took up arms in early April, refusing to recognise the legitimacy of a new national leadership they charge is made up of ultra-nationalists and neo-fascists.
