The US has brought new manslaughter charges against four former Blackwater personnel for a 2007 shooting in Baghdad's Nisour Square that prosecutors say killed 17 unarmed civilians, including women and children.

Prosecutors say the heavily armed Blackwater convoy launched an unprovoked attack using sniper fire, machine guns and grenade launchers in the shooting that also wounded at least 20 people.

The shooting caused tension in US-Iraqi relations and raised concerns about the US government's use of private contractors, who were shielded from prosecution in Iraq.

The vast majority of the US contractors who served in Iraq did so with honour and integrity, but, as alleged today, these defendants abused their power through a relentless attack on unarmed civilians that recklessly exceeded any possible justification. US Attorney Ron Machen, Chief Prosecutor

The original US charges filed against the men in 2008 were thrown out in December 2009, about a month before a scheduled trial.

The dismissal outraged many Iraqis, who said it showed Americans considered themselves above the law. Vice President Joe Biden, speaking in Baghdad in 2010, expressed his "personal regret" for the shootings and declared that the US would appeal the court decision.

The case ran into trouble because the State Department promised the guards that their statements explaining what happened would not be used in a criminal case.

The guards told investigators that they fired their weapons, a crucial admission because forensic evidence could not determine who fired.

Because of a limited immunity deal, prosecutors had to build their case without those statements, a high legal hurdle.

The case was reinstated in 2011 and prosecutors began a lengthy review of what charges they could prove in court.

The new indictment returned by a grand jury in Washington charges 33 counts, including voluntary manslaughter, attempt to commit manslaughter and using a firearm in a crime of violence.

The men, Paul Slough, Nicholas Slatten, Evan Liberty, and Dustin Heard had pleaded not guilty to the nearly identical charges brought five years ago.

"We are disappointed that the Department of Justice has chosen to proceed with this prosecution, which we strongly believe has no merit whatsoever," Heard's attorney, Dave Schertler, said in a statement.

"The vast majority of the US contractors who served in Iraq did so with honour and integrity, but, as alleged today, these defendants abused their power through a relentless attack on unarmed civilians that recklessly exceeded any possible justification," US Attorney Ron Machen, the chief prosecutor in Washington, said in a statement.

A hearing in the case is scheduled for October 25. No trial date has been set, and a defence lawyer said at a hearing last month that a trial could be years away.

Prosecutors last month dropped their case against a fifth guard, Donald Ball.

They said they were exercising "prosecutorial discretion" based on their "assessment of the admissible evidence against him."

Blackwater is now named Academi and is based in McLean, Virginia.