At least 60,000 people have been killed in Syria's civil war, with monthly casualty figures steadily increasing since the conflict began almost two years ago, according to a new analysis.

The death toll calculated by the United Nations is one-third more than the figure of 45,000 given by activists opposed to the regime of President Bashar al Assad.

Independent experts compared 147,349 killings reported by seven different sources – including the government – for the study, which was commissioned by the UN human rights office.

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By removing duplicates they arrived at a list of 59,648 individuals killed between the start of the uprising on March 15, 2011, and November 30, 2012. In each case, the victim's first and last name, the date and the location of his or her death were known.

"Given there has been no let-up in the conflict since the end of November, we can assume that more than 60,000 people have been killed by the beginning of 2013," UN High Commissioner for Human Rights Navi Pillay said. "The number of casualties is much higher than we expected, and is truly shocking."

The real death toll is likely to be even greater because reports containing incomplete information were excluded and a significant number of killings may not have been documented.

"There are many names not on the list for people who were quietly shot in the woods," Ms Pillay said.

The data, which did not distinguish among soldiers, rebels and civilians, also show the killing in Syria has accelerated.

During the summer of 2011, shortly after the uprising against Mr Assad began, the monthly death toll stood at around 1000. A year later, an average of 5000 were killed each month, the UN said.

Most of the killings occurred in Homs, followed by rural Damascus, Idlib, Aleppo, Daraa and Hama. At least three-quarters of the victims were male.

"The failure of the international community, in particular the UN Security Council, to take concrete actions to stop the blood-letting, shames us all," Ms Pillay said. "Collectively, we have fiddled at the edges while Syria burns."

She warned thousands more would die or suffer terrible injuries if the conflict continues, and repeated her call that those responsible for the killings –which in some cases could amount to war crimes – should be held accountable.

"We must not compound the existing disaster by failing to prepare for the inevitable – and very dangerous – instability that will occur when the conflict ends," said Ms Pillay.

She added: "Serious planning needs to get under way immediately, not just to provide humanitarian aid, but to protect all Syrian citizens from reprisals and acts of revenge such as those seen in Afghanistan, Iraq, Somalia and Congo."