Hundreds of civilians and military personnel have been killed in recent weeks alone after an upswing in fighting particularly near Donetsk airport and in the Debaltseve area, the Geneva-based body said in a report covering the period from December to February.
The strategic railroad town of Debaltseve was captured from Ukrainian government forces last month by pro-Russian separatists.
While Russia denies its troops are fighting in Ukraine, the UN cited "credible reports (that) indicate a continuing flow of heavy weaponry and foreign fighters" from Russia.
UN rights chief Zeid Raad al-Hussein said many civilians stay in embattled areas "because they fear for their lives if they try to move."
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"Many others stay to protect children, other family members, or their property," while some are forced to stay or unable to leave, he said.
The report cited "credible allegations of arbitrary detention, torture and enforced disappearances, committed mostly by the armed groups but in some instances also by the Ukrainian law enforcement agencies." It noted video footage appeared to support allegations of summary executions by the rebels.