The US military says more than one-thousand US and Iraqi troops have entered the Baghdad district of Sadr City -- a Shi'ite Muslim stronghold, conducting what it called "clearing operations" with no reports of violence.
The military says the troops met little resistance as they searched houses for weapons.
The moves come three weeks into a security crackdown in Baghdad that has reduced, but not eliminated, sectarian violence. Sadr City is largely under the control of the Mahdi Army militia, which appears to have gone into hiding.
Speaking to American television CNN today, a US military official in Iraq,Lieutenant General Raymond Odierno said Iraqi and US forces have arrested more than 700 members of the Mahdi Army in the last few weeks.
He said the security crackdown will take a minimum of six to nine months.
Separately, Iraqi Prime Minister Nouri al-Maliki threatened today to expand the crackdown to "every inch of Iraq."
Mr. Maliki also said he plans changes to his cabinet within the next two weeks but he did not give details.
Throughout Iraq Sunday, officials say at least 11 Iraqis were killed by gunfire and bomb blasts.
In the southern city of Basra, British officials say Iraqi and British forces raided a government detention center and arrested five suspected terrorists. Coalition forces also found evidence of torture among prisoners there.
Also today, a senior American commander in Iraq, Major General Benjamin Mixon, called on Iraqi leaders, both Sunni and Shi'ite, to denounce what he called horrific acts of violence. He was speaking about Friday's execution of 15 Iraqi police offers by militants in Diyala province.