"Some 3,300 people have been killed ever since the start of fighting on January 3 between the (jihadist) Islamic State of Iraq and the Levant (ISIL) on one side, and (rebel) Islamist and other groups on the other," said the Syrian Observatory for Human Rights.
The deaths came "in car and (other) bomb attacks, suicide blasts and fighting," said the Britain-based Observatory, which relies on activists and other sources inside Syria.
Among the overall fatalities were at least 281 civilians, the majority of them killed by shelling and stray bullets, the Observatory said.
But 21 of them were executed in a children's hospital-turned-ISIL prison in the northern city of Aleppo, it said.
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And it said the jihadists executed a family of seven Kurds, beheading some of them, at a prison in the Aleppo countryside.
The Observatory has documented the deaths of 924 ISIL members and 1,380 rebels, including Islamists.
It also said more than 700 other fighters from both sides had died in battle but could not be identified, while 29 bodies were found in ISIL bases, most likely executed by the jihadists expulsion before their expulsion.