UN monitors in eastern Ukraine say they've recorded a steady rise in killing, tortures and abductions by pro-Russian armed groups.
UN Assistant Secretary-General for Human Rights Ivan Simonovic said "a climate of lawlessness prevails in the east."
According to a fresh report issued on June 18, at least 356 people, including 257 civilians, have been killed since May 7.
The report also noted there had been more than 200 cases of torture, and that 81 people were being held on June 7.
In the report, UN human rights chief Navi Pillay said that a "climate of insecurity and fear" has displaced 34,000 people, nearly half in Ukraine's eastern regions of Donetsk and Luhansk, which have declared independence from Kyiv.
Simonovic said the government reported that armed groups in the east had abducted 387 people in the east from April to June 7, including 39 journalists.
UN monitors, however, were able to confirm 222 abductions, including four people who have been killed, 137 who have been released and 81 who remain in detention.
Simonovic also noted a rise in reports of enforced disappearances and excessive use of force during Ukrainian government security operations "that have led to casualties among civilians, which we continue to investigate."
Simonovic told a briefing on the report at the International Peace Institute in New York on June 18 that representatives of the self-proclaimed Donetsk People's Republic have for the first time admitted that among their fighters are some volunteers from Russia, most from Chechnya or Cossacks.
Russian Foreign Ministry spokesman Alexander Lukashevich said the report presented a "one-sided and politically biased interpretation of events."
He called a recommendation that Ukrainian authorities should carry out anti-terrorist operations "in accordance with international standards" hypocritical and outrageous.
Simonovic said the UN is not "neutral" on human rights because it always sides with victims and against perpetrators.
Based on reporting by Itar-Tass and AP