Aleppo is divided between rebel groups in the east and government forces, bolstered by pro-regime militias, in the west.
IS has not had a presence in the city, but on Friday it boasted it had “reached the gates of Aleppo”.
“IS announced several times that it would launch an offensive on Aleppo without doing it. They were waiting for the right moment and took advance of Russian strikes on other rebels to advance,” said jihadism analyst Romain Caillet.
Thomas Pierret, an expert on Islam in Syria, said the US-led coalition bombing IS was “not very active” in Aleppo, and that Russia’s strikes there had hit mostly rebels, allowing IS to advance.
The coalition headed by Washington said on Friday it had conducted one air strike in Aleppo and one on Raqa the previous day.
Meanwhile, Iran’s Revolutionary Guards announced one of its senior commanders was killed by IS in the Aleppo area on Thursday. A Guards statement said General Hossein Hamedani had played an “important role... reinforcing the front of Islamic resistance against the terrorists” but was killed “during an advisory mission”.
The Observatory said Hamedani was the highest-ranking Iranian casualty of the Syrian conflict and that he died near Kweyris, a military airport in Aleppo province under siege by IS.
French Defence Minister Jean-Yves Le Drian said on Friday that “80 to 90 percent” of Russian strikes in Syria were aimed at propping up Assad.
He was speaking after French warplanes conducted new strikes overnight on IS targets in Syria, where Paris launched its first raids on September 27.
According to the Observatory, 16 IS Jihadists including three child soldiers were killed in the raids which hit “a training camp” on the southern edges of Raqa city.
The Russian air war has provided cover for Assad’s ground troops who have lost swathes of territory to Jihadists and rebel groups since 2011.
The campaign has been critical for the regime’s fight in Sahl al-Ghab, a strategic plain in Hama province bordering both its coastal bastion of Latakia and the rebel stronghold of Idlib province.