In a fresh report by its monitors in the conflict-torn country, the UN office also said hundreds had perished in fighting and that the rights situation had worsened since it took stock in early May.
"The main problem is in the areas controlled by armed groups," said Gianni Magazzeni, a senior official with the UN's Office of the High Commissioner for Human Rights (OHCHR).
"We have a situation of a reign of fear, if not terror, for those that are caught in those specific locations," he told reporters.
Since then, Magazzeni said, at least 356 people have been killed. The number was based on a mix of official data and research on the ground.
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A total of 86 of the dead were Ukrainian servicemen, including 49 whose transport plane was shot down on June 14.
Magazzeni said the figure of 270 non-military fatalities did not break down rebel fighters and civilian victims.
The rebels seized control of the strategic southern province of Crimea and various other mainly Russian-speaking communities in the east after the February ouster of Moscow-tied president Viktor Yanukovych.
Moscow rejects the criticisms, as well as charges of sending arms and men to the rebels in areas such as the city of Donetsk.