Islamist State militants killed at least 15 civilians in the group's former stronghold of Mosul on Tuesday, posing as security officials to entice their victims.
A local official cited by the New Arab says the jihadists wore police uniforms to encourage local people to come and welcome them, in one of the few areas of the city still under Isis control.
Civilians reportedly came and welcomed the apparent liberators, only to be shot dead.
The Joint Operations Command said in a statement that Isis "terrorist gangs committed a brutal crime yesterday morning in an area of Mosul's old city."
The militants, it said, wanted to "confound civilians who expressed joy and welcomed with chanting."
Hossameddin al-Aabbar, a member of the Ninevin provincial council, told AFP that at least 15 people were slaughtered and several had been arrested.
Mosul was once Isis's strongest bastion in Iraq, but the majority of the city now lies under government control following a fierce fightback by Iraqi security forces.
Just hours before the slaughter, Iraqi forces retook the huge Tenek neighbourhood, one of the largest in west Mosul, after a week of fighting.