The Islamic State of Iraq and the Levant (ISIL) has captured a string of villages near Aleppo in a lightning attack that brought them closer to the prized city in northern Syria, despite a major increase in Russian air strikes that Moscow insists are targeting the group.
The surprise advance on Friday, which brings ISIL to within a few miles of Aleppo, was the most significant in months, following what has largely been a summer of stalemate in the country's conflict.
The gains come amid a wave of intense Russian air strikes that have targeted fighters trying to topple President Bashar al-Assad, and a ground offensive by the Syrian army in the country's central region.
Moscow says it is targeting mainly ISIL fighters, but US officials and Syrian rebels have said the strikes have hit "moderate" fighters for the most part and are aimed at shoring up Assad's embattled government and troops.
Many of the rebel groups hit by the Russian air strikes are also at war with ISIL.
"Everyone who wants to fight the rebels says that they are coming to fight Daesh [ISIL] 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.
Halaby said the Russians have not carried out attacks against ISIL in Aleppo, which is split between government forces and various rebel groups. "They are trying to finish off the rebels," he said.
Other activists wondered where the US military was.
"Why didn't America attack Daesh fighters during their attack?" asked Rami Abdurrahman, director of the UK-based Syrian Observatory for Human Rights.
'Change their tactics'
Russian General Staff deputy chief Lieutenant General Igor Makushev said Russian warplanes conducted 67 sorties over the past day - a significant increase over previous days.
Makushev said the strikes killed 100 fighters and destroyed an ammunition dump near Aleppo.
"Militants are sustaining substantial losses under the Russian air strikes and are having to change their tactics, to scatter their forces, to carefully disguise themselves and hide in settlements," he said.
An international human rights group said on Friday that apparent Russian air strikes on the central town of Talbiseh on September 30 killed at least 17 civilians and wounded 72, adding that the incident should be investigated for possible violations of the laws of war.
Human Rights Watch (HRW) said local groups operating in the central Homs province said that fighter jets that they believed were Russian conducted strikes on the neighbouring towns of Zaafaraneh and Rastan, killing another 17 civilians.
US-based HRW said it is still investigating the two attacks.
The observatory said ISIL's surprise advance north of Aleppo, which began on Thursday night, is the most significant in months.
It said the fighters seized the villages of Tal Qrah, Tal Sousin and Kfar Qares north of Aleppo, Syria's largest city and its one-time commercial capital.
The group also seized a former army base known as the Infantry Academy that rebels captured from the Syrian army two years ago.
The base is located about 16km northeast of Aleppo city and about 1.6km away from a government-held industrial zone on the northern edge of Aleppo.
The areas were seized from rebel groups, including Ahrar al-Sham and the Levantine Front.
The advance highlighted the rapidly shifting terrain in Syria, where Russia and the United States are launching air strikes and lending support to opposite sides in the country's nearly five-year civil war.
The pro-ISIL Aamaq media outlet said the group seized six villages and other strategic positions in Aleppo province after battling armed opposition groups there.
It said ISIL opened its blitz with a surprise attack on the Infantry Academy, forcing rebels to withdraw after a number of their fighters were killed or captured.
It said ISIL then continued their advance toward nearby villages, killing and wounding rebels and seizing "qualitative weapons and ammunition" from them along the way.
Some opposition activists said ISIL took advantage of Russian air strikes and the rebels' preoccupation with fighting Syrian troops on other fronts in central Syria.
"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 that includes "moderate" rebels as well as al-Qaeda's affiliate, the Nusra Front.
The Army of Conquest, which had made a string of advances against government troops in recent months, has come under attack from Russian warplanes.
Meanwhile, an Iranian Revolutionary Guards Corps (IRGC) general has been killed near Aleppo, where he was advising the Syrian army on their battle against ISIL, the guards said in a statement on Friday.
The IRGC said General Hossein Hamadani was killed on Thursday night and that he had "played an important role ... reinforcing the front of Islamic resistance against the terrorists".
Iran is the main regional ally of Assad and has provided military and economic support during the country's four-year-old civil war.