Iraqi officials say at least 40 Shi'ite militiamen and 20 Iraqi soldiers have been killed in fierce fighting in the southern part of the country.
The fighting began late Sunday in the Shi'ite-majority city of Diwaniyah, and continued for much of Monday.
A U.S. military official in Baghdad, Army Brigadier General Dana Pittard said Iraqi security forces repelled militia attacks after what he called "some tough fighting."
Authorities say the fighting broke out as Iraqi security forces carried out raids to collect illegal weapons from militiamen of the Mahdi Army loyal to radical Shi'ite cleric Muqtada al-Sadr.
Prime Minister Nouri al-Maliki has vowed to disband militias as part of his national reconciliation plan.
Earlier Monday, a suicide car bomber blew himself up outside the Interior Ministry in Baghdad, killing 16 people and wounding more than 40.
Despite a spike in violence over the past two days, a U.S. military spokesman Major General William Caldwell said a security operation in Baghdad has reduced violence in the capital.
Violence in the southern city of Basra, the northern city of Kirkuk and other parts of Iraq on Sunday killed more than 60 people.
The U.S. military in Iraq says eight American soldiers also were killed Sunday in several attacks.