Of those, 33,278 were civilians, said the UK-based NGO, which has been monitoring the situation in Syria.
The prolonged conflict has claimed over 191,000 lives in its first three years, according to the estimates, provided by the United Nations in August 2014.
"Tragically, it is probably an underestimate of the real total number of people killed during the first three years of this murderous conflict," Navi Pillay, the then UN High Commissioner for Human Rights, said in a statement.
Fierce fighting between the Syrian army and Islamist militants, including foreign mercenaries and al-Qaeda-affiliated groups, have been ongoing in Syria since March 2011. Over 2.3 million Syrian citizens have been forced to flee the country; and some 6.5 million Syrians have been displaced within Syria.