The Pentagon said American aircraft attacked artillery that was being used against Kurdish forces defending the northern city of Irbil.
President Barack Obama authorised air strikes on Thursday, but said he would not send US troops back to Iraq.
The Sunni Muslim group IS, formerly known as Isis, now has control of large swathes of Iraq and Syria.
In June, IS took control of the city of Mosul. Earlier this week, its fighters seized Qaraqosh, Iraq’s biggest Christian town.
The advance of IS also forced tens of thousands of people from the Yazidi community – another minority group in northern Iraq – to leave their homes and seek shelter on a nearby mountain.
The United Nations says it is working on opening a humanitarian corridor in northern Iraq to allow stranded people to flee.