The Britain-based Syrian Observatory for Human Rights, a network of activists monitoring the conflict, said the extremist group took five villages in Azaz district, north of Aleppo, where a rebel enclave is home to tens of thousands of displaced civilians. The IS group's news agency also reported the advance.
Syrian rebels are anticipating a major government offensive in Aleppo, Syria's largest city and onetime commercial capital, which has been contested since 2012.
In another blow to the rebels, government troops repelled an attack on the western edge of Aleppo by a coalition of insurgent groups, killing at least 19 fighters, said Nazeer al-Khatib, an Aleppo-based activist who keeps close contact with the insurgents.
Al-Khatib said the rebels were ambushed by government forces near a factory, trapping nearly 600 fighters inside.
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Nine rebels were killed in the clashes, while another 10 died during an operation that succeeded in freeing the trapped fighters but failed to dislodge government forces from the area.
A pro-government TV station also reported that troops had repelled an attempt by insurgents to crawl up the city's sewage system in a western neighborhood. The Sama TV station aired footage of the bodies of nearly a dozen fighters.
A government offensive backed by Russian air power and regional militias earlier this year dislodged rebels from parts of Azaz and severed their corridor between the Turkish border and Aleppo.
The predominantly Kurdish Syrian Democratic Forces (SDF), who are fighting for their autonomy in the multilayered conflict, also gained ground against the rebels.
Doctors Without Borders and other aid organizations warned earlier this month that the humanitarian situation for over 100,000 people trapped in the Azaz rebel-held pocket was critical.