The attack was launched against the internment facility at Camp Bucca, the military said in a brief statement.
Lieutenant-Colonel Christopher Garver, a military spokesman in Baghdad, said the centre appeared to have been hit by rockets.
The US military oversees some 21,000 inmates at Camp Bucca in southern Iraq and Camp Cropper, near the Baghdad airport.
In Baghdad on Saturday, sporadic clashes erupted for nearly two hours in a predominantly Sunni neighbourhood in the west.
"Let the people of Iraq vote if they want the US to stay or leave"
Bob Kaye, Bohemia, US
Send us your views
The fighting began when Iraqi army patrols came under attack in Amil and returned fire, an Iraqi army official said.
The official said four civilians were wounded, but no other casualties were reported and the situation was brought under control after the US military provided support.
Elsewhere in Baghdad, a parked car bomb struck a convoy of Iraqi police commandos and fighters opened fire on police on a foot patrol in a mostly Shia neighbourhood, police said.
Also on Saturday, a US soldier was killed by small-arms fire north of Baghdad, the US military said.
One Task Force Lightning soldier died as a result of injuries sustained during operations in Diyala province.