"SOHR documented the death of 4,358 people, who were killed in December 2014," the UK-based monitoring group said in a statement posted on its website.
Among the documented deaths, 1,052 were civilians, of whom 156 were children under the age of 18. Over 600 were the deaths of anti-government rebels and Islamist fighters, while almost 700 deaths among government forces.
SOHR believes the death toll is at least 1,000 above the documented number due to the "absolute secrecy" on casualties and the difficulty of accessing the violent-prone areas.
The Syrian civil war began in the wake of the 2011 Arab Spring. Government forces have been fighting various rebel groups, predominantly the IS extremist militant group. The IS began fighting the Syrian government in 2012 and has since taken control of vast territories across the country.
According to the United Nations, the ongoing conflict has claimed some 200,000 lives and forced over 3.2 million Syrians to flee their homes.
