The US military confirmed an attack in the area and said that "coalition forces are responding to the incident".
Security forces continue to be the target of attacks by fighters in Iraq despite an overall decline in violence in the country this year.
The blast came as Gordon Brown, the British prime minister, was in Iraq on an unannounced visit, a day after the Iraqi cabinet drafted a law paving the way for British forces to withdraw in 2009, more than six years after the US-led invasion.
The law sets the end of May as the final date for combat operations and end-July as the withdrawal date.
Also on Wednesday, Iraqi police said a roadside bomb in the northern city of Mosul killed one civilian and wounded four others.