GENEVA (AFP) - At least 93,000 people, including over 6,500 children, have been killed in Syria's civil war, the United Nations said on Thursday, warning that the true death toll could be far higher.

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", Ms 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."

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," Ms 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.

The study was commissioned from the Human Rights Data Analysis Group, a non-profit organisation of statisticians, computer scientists, demographers and social scientists.

They used eight sources, including the government, rebels, and groups such as the British-based Syrian Observatory for Human Rights, which relies on a network of activists and medics on the ground and last month put the toll at more than 94,000.

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.