Human rights violations have escalated in eastern Ukraine and serious problems are emerging in Crimea, United Nations monitors said in a report.

The 34-strong UN monitoring mission's report, which covered April 2 to May 6 and which Russia criticised as politically motivated, said police and local authorities in eastern Ukraine connived in illegal acts and the takeover of towns by armed groups.

The UN findings echoed a statement published on Monday by the Organisation for Security and Cooperation in Europe, whose experts had identified "a significant number of serious human rights violations" during a visit to Ukraine in March.

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Ukraine is preparing to hold presidential elections and several candidates had reported intimidation.

"Those with influence on the armed groups responsible for much of the violence in eastern Ukraine (must) do their utmost to rein in these men who seem bent on tearing the country apart," UN human rights chief Navi Pillay said in a statement accompanying the report.