* U.N. issues first death toll in Syria in a year
* UN rights envoy criticises "international paralysis"
* Says toll is probably an underestimate
By Stephanie Nebehay
GENEVA, Aug 22 (Reuters) - More than 191,000 people were killed in the first three years of Syria's civil war, a U.N. report said on Friday, and the world body's human rights envoy rebuked leading powers for failing to halt what she branded a "wholly avoidable human catastrophe".
U.N. High Commissioner for Human Rights Navi Pillay said war crimes were still being committed with total impunity on all sides in the conflict, which began with initially peaceful protests against President Bashar al-Assad's rule in March 2011.
"It is a real indictment of the age we live in that not only has this been allowed to continue so long, with no end in sight, but is also now impacting horrendously on hundreds of thousands of other people across the border in northern Iraq, and the violence has also spilled over into Lebanon," said Pillay.
Pillay, in a statement issued a week before leaving office, added: "The killers, destroyers and torturers in Syria have been empowered and emboldened by the international paralysis.
"It is essential governments take serious measures to halt the fighting and deter the crimes, and above all stop fuelling this monumental, and wholly avoidable, human catastrophe through the provision of arms and other military supplies."
The report by her Geneva office was based on data from four rebel groups and the Syrian government. They were cross-checked to eliminate duplicates and inaccuracies, including non-violent deaths or alleged victims later found to be alive.
It said the number of men, women and children killed in the conflict as of April 30, 2014, totalled at least 191,369. Of them, some 62,000 - both civilians and combatants - were killed in the past year alone, Pillay's spokesman Rupert Colville said.
The figure is more than twice the number of deaths documented a year ago and is probably still an under-estimate, Pillay said.
WOMEN AND CHILDREN
Colville told a news briefing in Geneva that around 5,000 to 6,000 people were being killed on a monthly basis.
Men and boys account for the bulk of the deaths but nearly 18,000 women and more than 2,000 children under the age of nine are also among those killed, he said.
Assad's government supplied just one set of figures on killings to the United Nations in March 2012, Colville said.
"We consider their information important because it's a little bit of a different perspective and possibly different groups of people that they focus on," he said, adding they were "almost exclusively military or police".
All groups involved in the fighting -- including the government, the army, police, Islamist militants and other opposition groups -- have committed killings, Colville said.
The U.N. report said it had excluded from its analysis an additional 51,953 killings that were reported but lacked required information of full name, date and location of death.
A further "significant" number may not have been reported by any of the five sources, it added.
The highest number of documented killings were recorded in Rural Damascus province, Aleppo and Homs.
Pillay repeated her longstanding call on world powers on the U.N. Security Council to refer alleged war crimes and crimes against humanity committed by all sides in Syria's conflict to the International Criminal Court (ICC). (Editing by Gareth Jones)