The skyrocketing number of deaths over the past year, along with documented cases of children tortured and entire families massacred, "is a terrible reminder of just how vicious this conflict has become," UN rights chief Navi Pillay said.
Describing the killing as "senseless carnage", Pillay said the toll in a new UN-commissioned study was "most likely a minimum casualty figure. The true number of those killed is potentially much higher."
"This extremely high rate of killings, month after month, reflects the drastically deteriorating pattern of the conflict over the past year," Pillay said.
Nearly 27,000 people have been killed since December alone, she said.
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"Civilians are bearing the brunt of widespread, violent and often indiscriminate attacks which are devastating whole swathes of major towns and cities, as well as outlying villages," she said.
The figures in the study, running from the outbreak of the conflict in March 2011 to the end of April this year, compares with a UN toll of 60,000 in November and an estimate of 94,000 by a monitoring group.
UN chief Ban Ki-moon said the near 27-month war was taking an "unacceptable and unbearable" toll on youngsters, and today's study found that at least 6,561 children -- 1,729 of them under 10 years old -- had lost their lives.