A grand jury has convicted four former Blackwater private security guards of killing 17 unarmed Iraqi civilians in Baghdad in 2007.

The Washington jury found Nicholas Slatten guilty of first-degree murder, and Paul Slough, Evan Liberty and Dustin Heard guilty of at least three counts of voluntary manslaughter, as well as attempted manslaughter and weapons charges.

"This verdict is a resounding affirmation of the commitment of the American people to the rule of law, even in times of war," Ronald Machen, US Attorney for the District of Columbia, said in a statement.

"Seven years ago, these Blackwater contractors unleashed powerful sniper fire, machine guns and grenade launchers on innocent men, women and children. Today, they were held accountable for that outrageous attack and its devastating consequences for so many Iraqi families."

Judge Royce C Lamberth allowed the jury to deliver those verdicts that it had reached, and will be able to continue deliberating on those it was unable to reach a unanimous verdict on.

The company had been employed by the US State Department to protect US diplomats and other areas of Iraq during the US occupation of the country.

More than 71 Iraqi witnesses testified in the two-month trial, the largest number of foreign witnesses ever to travel to the US for a foreign trial, said Machen.

Slatten's murder charge carries a mandatory life sentence, while the manslaughter charges carry maximum sentences of 15 years each.

The attempted manslaughter charges carry a seven-year maximum sentence, and the firearms offences can carry 30-year maximum sentences.

Judge Royce ordered the men to be detained pending sentencing.

Over the course of the trial, defence lawyers argued that there was strong evidence the contractors had come under fire from Iraqi police and insurgents, and returned fire in self defence.

Prosecutors argued that the defendants had not been provoked to fire back.

They said the men harboured a deep resentment of Iraqi civilians, and fired at them indiscriminately.

Prosecutors told the jury that Slatten, a sniper, opened fire on a car of Iraqi civilians as a convoy protected by Blackwater guards in armoured vehicles was making its way through a traffic jam in Baghdad's Nasoor Square.

Other members of the convoy then opened fire indiscriminately, claiming that they believed they were under attack.

FBI investigators, who visited the scene afterwards, described the crime as the "Mai Lai of Iraq", in reference to the infamous slaughter of Vietnamese civilians by US troops during the Vietnam War.