Islamic State (ISIS) militants abducted about 900 Kurdish civilians in the northern Syrian province of Aleppo over the past three weeks, forcing the captives to build fortifications for the extremists in retaliation for a Kurdish-led assault on a nearby ISIS stronghold, activists said Friday.

Reports also emerged Friday that at least 26 of those abducted have been killed for refusing to follow ISIS orders.

The abductions come amid fierce fighting for control of Manbij — a key ISIS stronghold in this Syrian province — where the extremists are being routed from the town center by the predominantly Kurdish and U.S.-backed Syria Democratic Forces.

Some of the abducted Kurds have been pressganged into digging trenches and shelters for the IS, according to Kurdish media activist Rezan Hiddo, while others have been forced to wear IS uniforms and ordered to fight at the fronts.

On Friday, the mostly Kurdish SDF battled IS militants inside Manbij after having encircling the stronghold in a weeks-long offensive that has been backed by U.S.-coalition air strikes.

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