Iraqi Kurdish forces and Shiite militiamen discovered mass graves containing 35 bodies after retaking the town of Sulaiman Bek from jihadists, an officer and a doctor said on Friday.
It was not clear when the shooting deaths took place, as the town north of Baghdad has fallen from government control several times this year, most recently from June until earlier this week.
The 35 bodies were taken under guard in ambulances to the morgue in Kirkuk, a city to the north of Sulaiman Bek, so their identities could be determined, Doctor Baha al-Bayati said.
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At least some of the bodies were badly decomposed, and the stench of rotted flesh still permeated the air in the town, where freshly-churned earth and nearby shovels marked one of the sites where they were exhumed, an AFP journalist said.
A Kurdish officer said an area of bloodstained ground near the entrance to the town was apparently where the killings took place.
Militants led by the Islamic State (IS) jihadist group launched a major offensive in June, overrunning chunks of five Iraqi provinces and sweeping security forces aside.
Iraqi forces, militiamen and Kurdish fighters broke a months-long jihadist siege of the town of Amerli on Sunday and retook Sulaiman Bek the following day, in the first major offensive successes for the federal government since the crisis began.
IS holds significant territory in neighbouring Syria in addition to Iraq, and has declared a cross-border Islamic "caliphate" in which it has carried out atrocities that have shocked the world.