The Syrian Observatory for Human Rights could not give a specific toll, but said the majority of those killed since July 31 were rebels and jihadists "because of the aerial superiority of the regime and intense Russian air strikes".
Observatory chief Rami Abdel Rahman said at least 130 civilians had also been killed since Sunday, most in rebel shelling of government-controlled districts.
He said at least seven civilians were killed today in rebel shelling of the regime-held neighbourhood of Hamdaniyeh.
The offensive has brought together rebels, Islamists and jihadists in an attempt to cut off a regime route into Aleppo and break the government siege of eastern districts.
Abdel Rahman said the advance left the regime forces "in a very difficult position despite Russian air support".
"This is an existential battle. Whoever wins it will win Aleppo," he said.
Repeated attempts particularly by Moscow and Washington to put an end to the fighting in Syria's battered second city have failed.
Russia and the United States support opposing sides in Syria's civil war, which has left 280,000 people dead and forced half the population to flee their homes.
On Thursday, US President Barack Obama said Russia risked casting itself as an "irresponsible actor" on the world stage, particularly over its actions in Aleppo in recent weeks.
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