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 on Thursday.

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."

She urged the global community to step up peace efforts to end a conflict that first began as peaceful demonstrations demanding reforms in the government of President Bashar al-Assad but after intervention of US and its western and regional allies it turned to a massive bloody all-out war.

"I urge the parties to declare an immediate ceasefire before tens of thousands more people are killed or injured," she said. "Nobody is gaining anything from this senseless carnage."

The number of people killed has skyrocketed over the past year, with the average monthly toll since July last year 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.

Nearly 27,000 people have been killed since December alone, she said.

"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.

On Wednesday, another grim UN report said Syrian children were being used as sniper targets and human shields, as well as facing torture and recruitment as fighters.

UN chief Ban Ki-moon said the near 27-month war was taking an "unacceptable and unbearable" toll on youngsters, and Thursday's study found that at least 6,561 children -- 1,729 of them under 10 years old -- had lost their lives.

"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.

The UN team crosschecked 263,055 killings, before excluding double-counting and some 38,000 deaths for which either a name, date or location were lacking, giving a total of 92,901.

"This total is likely to underestimate the actual number of killings," the UN rights office said, adding that "at least" 93,000 must have died and that there was "a strong likelihood that a significant number" may have gone unreported.

The study showed the hardest-hit regions were the rural district around Damascus, with 17,800 deaths, Homs, where 16,400 were documented, Aleppo, with 11,900, and Idlib, with 10,300.

In Daraa, there were 8,600 deaths, followed by Hama with 8,100, urban Damascus with 6,400 and  Deir Ezzor 5,700.

The sharpest increases since 2012 were recorded in rural Damascus and Aleppo, with 6,200 and 4,800 new documented deaths respectively.