In addition, nine U.S. soldiers were killed in explosions north of Baghdad, the military said Tuesday.
Six soldiers died Monday when a bomb exploded near their vehicles in Salahuddin province. Three others died the same day when another roadside bomb detonated in Diyala province northeast of Baghdad, according to the Associated Press.
Tuesday's suicide bombers struck on a main street in Hillah, 60 miles south of Baghdad, said Capt. Muthana Khalid. The street was lined with tents offering food and rest areas.
The attacks came a little more than a year after the bombing of a Shiite shrine in Samarra, which sparked a wave of sectarian violence that threatened to escalate into civil war.
The bombings also followed the beginning stages of a U.S. operation aimed at increasing the number of troops in Baghdad by 20,000 to crack down on violence there.
Iraqi Prime Minister Nouri al-Maliki called the coordinated bombing "a message to the world that the terrorists oppose humanity and knowledge," quoted Reuters.
At least 24 other Iraqis, including eight Shiite pilgrims, were killed in southern Baghdad Tuesday when gunmen fired into their minibus.
Bahaa al-Araji, a Shiite parliament member, said the incident pointed to "some shortcomings" in the Baghdad security plan. "The government bears some responsibility for this because it has not provided enough security forces to protect the pilgrims," he said, Reuters reported.