Investigation ordered after British-Iraqi raid on intelligence agency in Basra.
US officials announced earlier that nine American soldiers were killed and four wounded in two separate attacks in northern and north-eastern Iraq, the US military has said.
“Six Task Force Lightning soldiers were attacked while conducting combat operations in Salah ad Din province on March 5,” a statement said on Tuesday.
In a second incident on the same day, three American soldiers were killed by a roadside bomb in Diyala, one other was wounded.
Task Force Lightning
The US army’s Task Force Lightning is based in Tikrit, the former hometown of Saddam Hussein, the executed Iraqi dictator, and its patrols cover a wide area of northern Iraq rife with armed Sunni groups.
The deaths brought the US military’s losses in Iraq since the March 2003 invasion to 3,182, according to an AFP tally based on Pentagon figures.
Iraqi and US forces are in the third week of a security crackdown in Baghdad aimed at stemming sectarian violence.
US military commanders had expressed concerns that fighters could step up attacks outside Baghdad while the operation was under way in the capital.