The Iraqi army, Shia militias and Kurdish Peshmerga have joined forces to try to free the town of Amerli in northern Iraq, local sources say.
Some 15,000 minority Shia Turkmen in Amerli have been under siege by Islamic State (IS) militants for two months.
The UN called for urgent action last week to stop a massacre in the town, which lies in Kurdish-controlled Iraq.
Islamic State jihadists have been accused of atrocities in areas of Iraq and Syria under their control.
The Shia Turkmen are seen as apostates by the IS militants.
The BBC's Jim Muir, in the city of Irbil, says the combined forces are mounting an assault on two fronts in the Salahuddin Kurdish area in northern Iraq.
Kurdish Peshmerga fighters are said to be west of Tuz Khurmatu, which lies just north of Amerli, while Iraqi army units and Shia militia are approaching Amerli from the south.
The operation is backed by the Iraqi Air Force which is providing some cover on the southern approach, but there has been no back-up by US forces, local sources told our correspondent.
The operation is reported to have two objectives: to break the siege of Amerli and to reopen the main highway leading north from Baghdad.
The road is currently blocked by Islamic State forces.
Meanwhile reports from Syria say that hundreds of Yazidi women, another Iraqi minority, have been sold and distributed as wives among militant fighters in Syria.
The women who were abducted during recent attacks by IS in Iraq are said to have been transported to Syria after being forced to convert to Islam.
At least 27 of them were sold to IS members for marriage, according to the UK-based monitoring group the Syrian Observatory for Human Rights.
The US had supported Iraqi and Kurdish forces with air strikes against IS militants to retake the strategically important Mosul Dam earlier in August, but has since scaled back its military operation after President Obama admitted there was no defined US strategy yet in Iraq.