Islamic State fighters pushed back rival insurgents north of Aleppo on Sunday near the Turkish border, threatening their supply route to the city, fighters and a group monitoring the war said.
Fighters from Levant Front, a northern alliance which includes Western-backed rebels and Islamist fighters, said they were worried Islamic State was heading for the Bab al-Salam crossing between Aleppo and the Turkish province of Kilis.
"Reports indicate that the regime is making air strikes in support of ISIL's advance on Aleppo, aiding extremists against the Syrian population," a post on the U.S. Embassy Syria Twitter account said late on Monday, using an acronym for Islamic State.
Syrian officials have previously dismissed as nonsense allegations by Washington and Syrian opposition activists that the Syrian military has helped Islamic State's fight against rival Syrian insurgent forces.
"The Syrian army is fighting Islamic State in all areas where it is present in Syria," a military source said.
