The Syrian Observatory for Human Rights, which relies on a network of activists in Syria for its information, said the death toll was 100,191, thousands more than the latest U.N. estimate of at least 93,000 by the end of April, The New York Times reported.
Syrian Observatory's report said pro-government forces had more casualties than rebels, while civilian deaths accounted for a third of the toll.
Rami Abdul Rahman, the organization's founder, told the Times in a phone interview statistics were compiled by totaling daily tallies his organization has kept since the beginning of the civil war.
In its breakdown, the group said the dead included 36,661 civilians, 13,539 rebel fighters and 2,015 defectors. The group said 25,407 regular soldiers had been killed along with 17,311 members of militias and pro-government units.
"The death toll does not include more than 10,000 detainees and missing persons inside of regime prisons, nor does it include more than 2,500 regular soldiers and pro-regime militants held captive by rebel fighters," the Syrian Observatory said on its website.
"We also estimate that the real number of casualties from regular forces and rebel fighters is twice the number documented, because both sides are discreet about the human losses resulting from clashes," the statement added.