BEIRUT -- Its often described as a slaughter by the regime of Bashar Assad, an endless series of brutal and heartless massacres, but the protracted, two-and-a-half year old war in Syria has left tens of thousands of casualties on all sides of the fight, a recent tally by a leading monitoring group has found.

The newest data , from the Britain-based Syrian Observatory for Human Rights, find that of the more than 100,000 people who have died in the fighting, 21,850 have been rebel fighters and 45,478 have come from the pro-regime side (27,654 from the regular army and 17,824 from semi-official, pro-regime militias). The group also counted more than 40,000 civilians among the dead.

The Syrian Observatory, which relies on a network of personal contacts throughout the country and says it only verifies a death when it has more than one source, has consistently been one of the most reliable tabulators of casualties in the Syrian conflict.

The organization is also generally sympathetic to the uprising, making some of its recent assessments -- including its finding of so many more regime casualties than rebel ones -- all the more telling about the character of the grueling war.

In August, after the U.S. declared that 1,429 people had been killed in an apparent chemical weapons attack in eastern Damascus, the Syrian Observatory responded that it had only confirmed 502 casualties, and could not comprehend where the American figures came from.

They always say Assad killed one hundred thousand people," scoffed Rami Abdul-Rahman, the director of the Syrian Observatory, by phone. How is this possible? Is he killing his own army and his own militias?

Analysts have expressed surprise at the ratio of deaths as determined by the Syrian Observatory, although they don't dispute the basic premise that the war has taken a severe toll on all sides.

It's good that these statistics bring home the point that there's a war going on, said Aron Lund, an independent researcher who has closely tracked the course of the Syrian conflict. "The exact proportions seem a little strange," he added, noting that one might expect the disorganized insurgent side of a conflict to suffer higher casualties than a structured army.

A recent United Nations report found that in the case of civilian massacres, a vastly disproportionate number of confirmable instances have come at the hand of pro-regime forces.

But the U.N. also found at least one episode of a massacre by rebel fighters. Many civilian casualties come as the result of indiscriminate shelling of residential areas, something that both parties in the war are guilty of.

Abdul-Rahman, for his part, defends his methodology, noting that in several cases -- and to a growing extent over the past year -- clashes between the rebels and the regime have come in the form of mass-casualty suicide bombings or the storming of military fortifications, which can lead to a high ratio of army-to-rebel deaths.

In the past few days, for instance, the Syrian Observatory has identified one attack on a military base in Daraa that left 27 soldiers killed and only 9 rebels, and a suicide bombing by the al Qaeda-aligned rebel group Jabhat al Nusra that left dozens of soldiers dead.

Abdul-Rahman told HuffPost that he is aware of at least one civilian massacre by rebel fighters, a mass killing of pro-Assad Allawites, but he has yet to confirm it enough to release the details publicly.