Militants from the Islamic terror group ISIS are gaining ground in Syria and continue to encroach on Aleppo, the country's largest city.

They have already taken control of large parts of homonymous province surrounding the highly symbolic locale, Agence France-Presse reported. Defying Russian airstrikes that Moscow claims are targeting the jihadis, ISIS fighters on Friday advanced to the city's outskirts and are now just six miles from the northern edges of the provincial capital.

ISIS controls large swaths of territory across Syria and Iraq, but Aleppo is currently divided between two of its enemies -- government forces loyal to Syrian dictator Bashar al-Assad, along with pro-Assad militias in the west, and Syrian rebel groups in the east.

"Dozens of combatants were killed on both sides," Rami Abdul Rahman of the Syrian Observatory for Human Rights said. "(ISIS) has never been so close to the city of Aleppo, and this is its biggest advance towards."

The surprise advance marked some of the ISIS' biggest gains in recent months as the terrorist group expelled Syrian rebels from at least five villages in Aleppo province, the Washington Post reported.

The offensive also led to the death of a high-ranking Iranian general, who was killed by ISIS supporters, the Wall Street Journal reported based on Iranian state media. Under what circumstances Brig. Gen. Hossein Hamedani lost his life on Thursday was not revealed, the newspaper added.

Iran and Russia have been among the strongest backers of Assad, who in 2011 plunged his country into a bloody civil war to avoid his outer, according to the Washington Post.

ISIS, meanwhile, may have taken advantage of the rebels' preoccupation with fighting pro-Assad forces, as well as of Russia's airstrikes, which are believed to frequently have targeted dissidents instead of jihadists, according to The Associated Press.

"At a time when the rebels are waging fierce battles against the Russian occupiers in Hama countryside, Daesh seizes the Infantry Academy and a number of villages in Aleppo," said Hadi Abdullah, an activist with close links to the Army of Conquest,  a coalition of mainstream rebels and a local al-Qaida affiliate.