The skyrocketing death toll, 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 in a statement.
Describing the killing as "senseless carnage" Pillay said that the UN's latest toll figure "is most likely a minimum casualty figure. The true number of those killed is potentially much higher."
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The number of people killed in the two-year conflict has skyrocketed over the past year, with the average monthly toll since July 2012 standing at more than 5,000, compared with 1,000 in the summer of 2011, the study said.
"This extremely high rate of killings, month after month, reflects the drastically deteriorating pattern of the conflict over the past year," Pillay said, adding that nearly 27,000 people have been killed since December 2012 alone.
"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 added.
The study, running from the outbreak of the conflict in March 2011 to the end of April this year, updates the toll of 60,000 which the UN gave in a November 2012 document.
The latest study underlined the extent to which the violence has spiralled since the start of the uprising against President Bashar al-Assad, which began as peaceful protests and degenerated into a civil war.
Pillay slammed both sides, pointing to government shelling and air attacks on urban areas, and the rebels' pounding of residential areas, albeit with less fire power, and bombings in the heart of cities, notably the capital Damascus.
Some 82.6 percent of the documented victims were male, while 7.6 percent were female, and the gender was not indicated in the remaining cases.
The analysis was not able to differentiate consistently between combatants and non-combatants, and around three-quarters of the reported killings did not record the victim's age.
But the deaths of at least 6,561 children -- 1,729 of them under 10 years old -- were documented.
"There are also well-documented cases of individual children being tortured and executed, and entire families, including babies, being massacred -- which, along with this devastatingly high death toll, is a terrible reminder of just how vicious this conflict has become," said Pillay.
"I urge the parties to declare an immediate ceasefire before tens of thousands more people are killed or injured," she said, urging the international community to step up peace efforts.
"Nobody is gaining anything from this senseless carnage."
