The surprise advance that brought the IS group to within a few miles of Aleppo was the most significant in months, following what has largely been a summer of stalemate in the war against the extremists.
It comes amid a wave of intense Russian airstrikes that have targeted insurgents fighting to topple President Bashar Assad, and a ground offensive by the Syrian army in the country's central region.
"Everyone who wants to fight the rebels says that they are coming to fight Daesh while in fact they want to fight the rebels," said Bahaa al-Halaby, an activist based in the city of Aleppo, using an Arabic acronym for the group.
He said the Russians have not carried out attacks against IS in Aleppo, which is split between government forces and various insurgent groups. "They are trying to finish off the rebels," he said.
Russian General Staff deputy chief Lt. Gen. Igor Makushev said Russian warplanes conducted 67 sorties over the past day a significant increase over previous days.
He said the strikes killed 100 militants and destroyed an ammunition dump near Aleppo.