Four former employees of the Blackwater security firm went on trial in the US capital on Wednesday, seven years after allegedly killing at least 14 Iraqi civilians in Baghdad.

Backed by an army of lawyers, Paul Slough, Evan Liberty, Dustin Heard and Nicholas Slatten appeared before a federal court for the start of jury selection.

The trial was expected to last between five and six months, Slatten's lawyer, Thomas Connolly, told AFP.

Judge Royce Lamberth said "a very high number of Iraqi witnesses" are scheduled to testify about what they saw on Sept 16, 2007, in Baghdad's Nisour Square.

The Blackwater employees were guarding a US diplomatic convoy when they opened fire, killing 17 unarmed Iraqi civilians according to an Iraqi investigation, or 14, according to the US count. The hail of gunfire also wounded 18 people.

The killing exacerbated Iraqi resentment toward Americans and was seen by critics as an example of the impunity enjoyed by private security firms on the US payroll in Iraq.

Slatten is charged with the first degree murder of a civilian. Slough, Liberty and Heard are accused of voluntary manslaughter of the 13 other victims.

All four have pleaded not guilty.

Their trial follows a tortuous legal process as the case slowly wound through US courts.

In 2009, a US judge dismissed charges against five former Blackwater employees because certain statements they made immediately after the event could not be used against them.

Two years later, an appeals court reinstated the indictments against four defendants, opening the way for the current trial.

But a judge dismissed the case against Slatten in April because of a technicality. Federal prosecutors then re-filed first degree murder charges against him several weeks later.

Slatten, if convicted, faces a maximum penalty of life in prison but prosecutors would have to convince a jury that he acted with premeditation.

Before the killings, Slatten was alleged to have told acquaintances he wanted "to kill as many Iraqis as he could as 'payback for 9/11'", according to the indictment.

Blackwater, whose license to work in Iraq was revoked by Baghdad, was renamed Xe Services in 2009 and then Academi in 2011.

On President Barack Obama's arrival in office in 2009, the State Department canceled its contract with the company.