In a new report, the UN's rights chief catalogued a litany of "targeted killings, torture and beatings, abductions, intimidation and some cases of sexual harassment" which it said was carried out by anti-government groups in the east.
With the May 25 vote rapidly approaching, Kiev's interim leaders are battling to keep Ukraine from disintegrating further after Russia's annexation of Crimea in March.
Government forces are pressing on with a military offensive to put down the bloody rebellion in Ukraine's industrial belt where well-armed insurgents have already seized over a dozen towns and cities in just a few weeks.
In a veiled reference to Russia, UN High Commissioner for Human Rights Navi Pillay called on those with influence on the armed groups in the east "to do their utmost to rein in these men who seem bent on tearing the country apart".
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She also voiced deep concern about "serious problems" of harassment and intimidation facing the Tatar community in Crimea, which was taken over by Russia in the face of international outrage.
"The complete lack of objectivity, blatant discrepancies and double standards leave no doubts that (the report's) authors were performing a political put-up job aimed at clearing the name of the self-declared authorities in Kiev," the foreign ministry said.
In Kiev, the Polish and Swedish foreign ministers were holding talks with Ukrainian leaders to pledge their backing for the vote, seen by the West as key to preventing the country from descending into all-out war.