The Britain-based Syrian Observatory for Human Rights, which has a network of activists around Syria, said that the fighting in northern and eastern Syria killed 1,069 since the clashes began January 3.
The fighting pitting the Al-Qaeda-linked Islamic State of Iraq and the Levant and other groups are the most serious among rebel forces since the Syrian conflict began in March 2011.
The Observatory said that the dead included 130 civilians including 21 who were "executed" by "Islamic State" members.
UN High Commissioner for Human Rights Navi Pillay said that over the past two weeks her office has received reports of "a succession of mass executions of civilians and fighters who were no longer participating in hostilities in Aleppo, Idlib and Raqqa by hard-line armed opposition groups in Syria, in particular by the" Islamic State of Iraq and the Levant.
She warned that such executions violate international humanitarian law, and the numbers of such violations are thought to be alarmingly high.