A US Marine at the centre of the biggest prosecution of US troops in the Iraq war should be tried for the lesser offence of negligent homicide instead of murder, an investigating officer said.

A US Marine Corps squad leader has been charged with murdering 12 people and ordering other Marines under his command to kill six people during an incident that left 24 civilians dead in the Iraqi town of Haditha last year.

A US politician says about six Marines will be charged in the killing of 24 civilians in the Iraq town of Haditha.