CAMP PENDLETON, California (AFP) - A US Marine was ordered to execute a room full of terrified Iraqi women and children during an alleged massacre in Haditha that left 24 people dead, a military court heard Thursday.

The testimony came in the opening of a preliminary hearing for Marine Sergeant Frank Wuterich, who faces 17 counts of murder over the Haditha killings, the most serious war crimes allegations faced by US troops in Iraq.

Wuterich, dressed in desert khakis, spoke confidently to confirm his name as the hearing to decide if he faces a court martial began at the Marines' Camp Pendleton base in southern California.

"He said 'Just wait till they open the door, then shoot,'" Mendoza said. Mendoza then said he himself shot and killed an adult male who appeared in a doorway.

"I looked at them for a few seconds. Just enough to know they were not presenting a threat ... they looked scared."