Meanwhile, an Arab League official predicted 100,000 would die in the conflict in 2013, CNN reported.
The human rights group, which opposes Syrian leader Bashar Assad, said of all the people believed killed in the civil war, 39,520, or 85 percent, died in the past year. The group said the early hours of 2013 saw fighting and shelling in eight Syrian provinces, with the heaviest fighting in Aleppo and Damascus and its suburbs.
The group said 6,548 were killed in the civil war in 2011.
But U.N.-Arab League envoy U.N.-Arab League envoy Lakhdar Brahimi said Sunday the death toll could rise dramatically.
"Do not expect just 25,000 people to die next year -- maybe 100,000 will die," Brahimi told reporters in Cairo, CNN reported.
Syria uses the Western calendar, and observed New Year's Day on Tuesday with a national holiday.