The American soldier died at about 4pm when a roadside bomb hit his convoy southwest of Baghdad, the military said.
April has been the deadliest month for American forces in Iraq this year with 70 servicemen killed.
Some US officers have suggested the increase in deaths could be due to better weather, making attacks easier.
Meanwhile, the Iraqi vice president, Adil Abd al-Mahdi, said that 90% of Iraq's growing number of displaced people were Shias like himself.
Other estimates of the number of displaced families have been lower.
Dr Salah Abd al-Razzaq, spokesman of the Shia Endowment, a government body that runs Shia religious institutions, put the number of displaced families at 13,750 nationwide, or about 90,000 people.
That includes 25,000 Iraqis who have fled their homes since an attack on a Shia mosque in Samarra on February 22 that triggered a wave of sectarian attacks on Sunni mosques and religious leaders.
Earlier this week, Major General Rick Lynch, a US spokesman, said US forces had found no "widespread movement" of Shias and Sunnis away from religiously mixed areas, despite reports to the contrary by Iraqi officials.
In Saturday's worst violence involving Iraqis, the bodies of six handcuffed, blindfolded and tortured men were found in the Baghdad neighborhood of al-Dura, said police Captain Jamil Hussein. The area has seen frequent sectarian violence.
Also, gunmen kidnapped a Sunni policeman and his brother from their home in the Sunni-dominated town of Jurf al-Sakhar and shot them to death, said police.
The town, 70km (43 miles) south of Baghdad, is near the mostly Shia city of Musayyib.