Militants with the Islamic State group seized several towns in northern Syria's Aleppo province early Wednesday, dealing a blow to rival rebel factions who were forced to withdraw from areas they took this year, according to fighters reached near the front line.
With the capture of the strategic town of Akhtarin and a few surrounding villages, fighters with the breakaway Al Qaeda group have moved farther west and now threaten to cut off the rebels' main access highway to neighboring Turkey.
The Islamic State's rivals are fighting on two fronts as they try to fend off a government attempt to seize control of the city of Aleppo.
Since capturing large swaths of territory in northern Iraq and declaring a caliphate in June, the extremist fighters have been making gains in neighboring Syria as well, benefiting from U.S. weapons seized in Iraq, as well as the chaos that more than three years of civil war have wrought in Syria.
Rebels fighting to oust Syrian President Bashar Assad this year expelled Islamic State fighters from parts of northern Syria.
Aleppo is not the only region where local fighters are trying to stand in the way of Islamic State militants.
In the oil-rich eastern province of Dair Alzour, members of the Shaitat tribe recently took up arms against Islamic State insurgents. The militants had consolidated control over much of the area, forcing members of Al Qaeda-affiliated Al Nusra Front to pledge allegiance to their leader, Abu Bakr Baghdadi, or be expelled from the area.
Islamic State fighters killed dozens of tribe members last week, according to photos distributed widely on social media.
The opposition Syrian Observatory for Human Rights said the Islamist extremists have also taken over three of the tribe's villages. But some of those who rose up against the insurgents reportedly have fled to neighboring areas, suggesting the backlash against the Islamic State could continue in Dair Alzour.
Islamic State controls about a third of Syria, according to the Syrian Observatory.
Rebels in Aleppo had hoped to stave off the western expansion of the group, which is trying to establish a regional caliphate governed by its extreme interpretation of Islam.
For more than a month, fighters with the rebel Islamic Front and Nusra Front have been battling the Islamic State along a front line that stretches from the city of Aleppo to the Turkish border. But they have said they will not be able to hold off the extremists without military aid and reinforcements.
Rebels not linked to Al Qaeda have said they received some aid from the Western-backed opposition Supreme Military Council, but not enough to counter all the weapons that Islamic State acquired recently by seizing military bases in Iraq and Syria.
"If we don't get more military aid soon, it is impossible for us to hold them off," Badee Muhammed, an Islamic Front commander, said weeks before Wednesday's defeat.
About 50 Islamic Front and Al Nusra fighters were killed in the latest clashes, which included heavy tank shelling and mortar fire, said a member of the Islamic Front who goes by the nickname Abu Mahmood Nasir. Several civilians were also killed, he said.