The military said in a statement on Saturday that the incident was under investigation.
Earlier on Saturday, the military announced that a US marine was killed in action on Friday in Iraq's al-Anbar province.
The soldier with the 1st Marine Expeditionary Force "was killed in action yesterday, while conducting security and stability operations in al-Anbar province", the statement said.
It gave no further details about his death, saying "the release of more details about the incident could place our personnel at greater risk".
Al-Anbar includes the towns of Falluja and Ramadi, west of Baghdad.
The latest death took to 1518 the number of US military personnel killed in Iraq since the invasion of the country just over two years ago, according to a toll compiled from Pentagon figures.