The U.S. army has charged three soldiers with murder in the killings of four Iraqis last year .

Military investigators say the Iraqi men were captured by an American patrol after a gun battle near Baghdad in April 2007. The captives were bound, blindfolded, shot in the head and dumped in a Baghdad canal, witnesses said in a preliminary hearing last month.

The killings are alleged to have been retribution for casualties suffered by U.S. forces.

The U.S. army said in a statement released Wednesday that the three soldiers face charges of premeditated murder, conspiracy to commit pre-meditated murder and obstruction of justice.

Four of their comrades have also been investigated in the case, of whom two have been charged.

A court-martial for one of the associated cases is scheduled to begin Thursday, but no date has been set for the other case. The two who have not been charged appeared before a military judge last month, but a decision on whether to court-martial them has not been taken, prosecutors said. Defence lawyers argued at their hearing that they did not participate in the killings, and the judges' ruling on their file is pending.

All seven are being held at a U.S. military detention facility in Iraq.

The case is the latest of several high-profile U.S. military prosecutions of soldiers who, it is alleged, attacked captives or civilians after a violent incidents involving their platoon.

In the most well known, eight U.S. Marines faced various charges in connection with the killing of 24 Iraqi civilians in 2005 in what was characterized in court as a frenzied revenge attack for the bombing of a convoy.

Subsequently, seven of the Marines were found not guilty by courts-martial or had charges against them reduced. None was charged with murder.

With files from the Associated Press