A former guard at the Blackwater military security firm has been sentenced to life in jail for his role in the killing of 14 civilians in Iraq eight years ago.

Nicholas Slatten was sentenced by US District Judge Royce Lamberth after a day-long hearing in Washington DC , despite defence lawyers' pleas for leniency. Three other guards - Paul Slough, Evan Liberty and Dustin Heard - were each sentenced to 30 years and one day in prison.

Slatten, who was accused of firing the first shots in the massacre, was convicted of first-degree murder, while Slough, Liberty and Heard were convicted on multiple charges including manslaughter and attempted manslaughter.

The four men opened fire on a throng of unarmed civilians in Nisour square, close to Baghdad's Green Zone, eight years ago. Reports from the scene suggest the men rampaged across the square while purportedly providing diplomatic protection.

The incident was christened 'Baghdad's bloody Sunday' and went down in infamy as the bloodiest attack on Iraqis by US government contractors during the 10-year conflict in the country.

When the four men were convicted last autumn, Attorney Ronald Machen said: "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."