Iraqi forces have moved into Baghdad's Sadr City neighborhood in an operation aimed at bringing government control over the Shi'ite militia stronghold.
Iraq security officials say they launched "Operation Peace" early Tuesday to try to stabilize and secure the area.
A military spokesman says hundreds of troops have deployed in several sectors of the city. He says no violence has been reported.
The operation is in line with a truce reached earlier this month between Iraq's government and Shi'ite militiamen loyal to radical cleric Moqtada al-Sadr.
Fighting in Sadr City has killed hundreds of people since late March, when Iraqi Prime Minister Nouri al-Maliki ordered a crackdown on Shi'ite militias.
In other news, Iraqi police say gunmen have killed at least four anti-al-Qaida fighters in an ambush in volatile Diyala province, north of Baghdad.
The gunmen were part of U.S.-backed Sunni groups that have turned against al-Qaida.
Also in Diyala today, the U.S. military says a mortar attack killed three Iraqis and wounded nine others. A separate statement says a suicide bombing killed a child and wounded two civilians in Jisr Naft in Diyala.
The U.S. military also says coalition forces killed a senior al-Qaida in Iraq leader in Samarra and detained 20 suspected terrorists during operations in Iraq over the past two days.
Some information for this report was provided by Reuters.