The Syrian Observatory for Human Rights (SOHR) said nearly half a million Syrians have been killed since the war erupted in 2011.
The rights body said in a new report marking the tenth anniversary of Syria's war, that it has documented the killing of 494,438 people since March 2011 when Syrian regime forces cracked down on peaceful protesters demanding political reform.
SOHR Director, Rami Abdel Rahman, told AFP that the recent unprecedented lull in the conflict has "provided us with a window to document tens of thousands of cases for which we lacked evidence", adding that "an overwhelming majority of these deaths occurred between the end of 2012 and November 2015".
The latest death toll includes 159,774 civilians, including more than 25,000 children who were killed as a result of military attacks by Syrian regime forces and their affiliated groups.
According to the report, the deaths include more than 168,000 Syrian regime soldiers and groups loyal to them, while 79,844 of deaths were recorded among armed opposition groups.
The organisation has documented the killing of 57,567 people in the Syrian regime's detention centres and prisons, while it is still documenting the deaths of more than 47,000 people due to torture and poor prison conditions, the report said.
Meanwhile, the fate of tens of thousands of people remains unknown after they went missing or were kidnapped.