Seven children, including twin baby girls, were among the dead, said the Syrian Observatory for Human Rights.
"The snowstorm that hit large swathes of Syria on Sunday caused the death of seven people," the Observatory said, updating an earlier toll.
Among the latest deaths was a two-day-old girl in the Firdous district of the northern city of Aleppo whose twin sister was reported dead on Sunday.
Signup to our newsletter and follow us on Facebook and Twitter!
"Both twins have now died because of the cold," Observatory director Rami Abdel Rahman told AFP.
The other new deaths included three boys: in Beit Sahem, south of Damascus, in the eastern town of Albu Kamal, controlled by the Islamic State jihadist group, and in southern Syria's Daraa province.
The fifth additional death was in Deir Ezzor province, which neighbours Iraq, where a man died because of a lack of heating, the group said.
All the victims lived in rebel-held areas, where shortages of food, heating and medical equipment are especially severe.
In neighbouring Lebanon, at least two Syrian refugees died last Wednesday due to plunging temperatures.