An army veteran of the Iraq war, Steven Green, 21, was charged by the US civilian authorities last week in the same case alleged to have taken place in the town of Mahmudiyah, south of Baghdad, on March 12.

Four of the five soldiers indicted Sunday were charged with rape and murder and the fifth with dereliction of duty for failing to report the offences.

The military stressed that the fifth soldier was "not alleged to have been a direct participant in the rape and killings."

Green, a former private with the US army's 101st Airborne Division, already pleaded not guilty before a civilian court in Louisville, Kentucky, on Thursday.

The military said the four "are charged with conspiring with former Private First Class Steven D. Green to commit these crimes."

It said those accused are "presumed innocent until and unless proven guilty beyond a reasonable doubt."

It said the next step in the legal process will be an "investigation pursuant to Article 32, Uniform Code of Military Justice" adding that Article 32 investigations are similar to grand jury hearings.

According to an affidavit filed last week along with the charges against Green, he and three other soldiers were accused of breaking into the house near Mahmudiyah and killing everyone inside.

The court documents, which describe the incident, said one victim was a young girl aged around five, while the rape victim was around 25 -- according to the alleged perpetrators.