More than 60,000 people have been killed since Syria's uprising began in March 2011, the UN has said.
A study commissioned by UN human rights commissioner Navi Pillay collated data from seven different sources, and concluded that there had been 59,648 deaths until November 2012.
Ms Pillay said the figure would now have risen above 60,000 and described the bloodshed as "truly shocking".
Syrian opposition groups had previously estimated 45,000 people killed.
The study was released hours after a petrol station near Damascus was hit by a missile from a government warplane.
The attack apparently sparked a huge explosion in which up to 70 people died, according to activists, in one of the deadliest incidents of the uprising.
The UN-commissioned report, entitled Preliminary Statistical Analysis of Documentation of Killings in Syria, took data from sources including the government and opposition groups.
The authors, from the Benetech research group, looked at 147,349 reports of deaths from all seven of its sources.
They cross-referenced the reports and removed duplicates, and also counted only those accounts that provided the first and last name of the victim, and the date and location of death.
After this process, they arrived at the figure of 59,648.
However, they warned that an unknown number of killings had not been documented by any of their sources.
"The statistics presented in this report should be considered minimum bounds," the report said.
Only four of the sources used covered the entire period.
Three are activist groups with links to the opposition: Violations Documentation Centre, Syrian Network for Human Rights, the Syria Shuhada website.
The other is the UK-based activist group Syrian Observatory for Human Rights.
The government's statistics were available only up to March 2012, and recorded only 2,539 deaths in total.
Also, the 59,648 figure includes many deaths reported by only one source.
For instance, in August 2012, the deadliest month of the uprising, the data shows just over 4,000 deaths that were corroborated on two or more sources.
Almost 2,000 other deaths were reported by only one source, but were still included in the final count.
The report also points out that the four data sources covering the entire period "appear increasingly to be sharing sources documenting killings".
The study did not provide a breakdown of whether those who died were rebels, soldiers or civilians.
But it highlighted that 76% of the reported victims were identified as male.
The worst-affected areas were Rural Damascus and Homs province.
Ms Pillay said: "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.
"The number of casualties is much higher than we expected, and is truly shocking."
She stressed that the deaths were caused by both sides in the conflict, and warned that increasing sectarianism was making it increasingly difficult to find a satisfactory solution.
Protests calling for modest reforms began in Syria in February 2011.
A series of clashes between security forces and demonstrators saw the rallies snowball into a full-on uprising against the rule of President Bashar al-Assad.
The rebels now control large sections of Syria, but the conflict has appeared to be largely in stalemate for months.