Sunday's attack followed a suicide bombing west of the capital on Saturday night that left at least nine people were killed, according to a local mayor.
The blast on Sunday came as the US military announced that May was the least deadly month for US troops in Iraq since the invasion in 2003.
Iraq's defence, interior and health ministries also announced at least 563 Iraqis were killed during May compared to at least 1,073 dead in April and 1,082 in March.