A grand jury has charged a former security guard for Blackwater, the US private security company, with murder for his alleged role in a 2007 shooting of unarmed civilians in Baghdad.

A federal appeals court last month effectively ended a manslaughter case against the guard, Nicholas Slatten, but prosecutors had signaled they might seek a new indictment against him.

The charge comes just weeks before three other former Blackwater guards are scheduled to face trial on manslaughter charges over the deaths of 14 Iraqis killed as the guards accompanied a State Department convoy through Baghdad's Nisur Square, the Reuters news agency reported.

The indictment, which was returned on Thursday but not made public until Friday, charges Slatten with the murder of one person, the driver of a white Kia sedan, which was the first shooting in the square.

Murder is a more difficult charge than manslaughter for prosecutors to prove because it involves proving that a defendant willfully and intentionally killed a victim.

The US Attorney's office in Washington, which is prosecuting the case, said it would seek to have Slatten tried along with the guards next month.

A lawyer for Slatten, Thomas Connolly, did not immediately respond to a request for comment.

Connolly had earlier said he would try to have any new indictment thrown out as a vindictive prosecution.

'Deep hostility'

The shooting, which came in the fourth year of the Iraq war, outraged Iraqis and further strained US-Iraqi ties.

Prosecutions related to the deaths have dragged on for years amid problems with evidence.

In court documents filed in March, prosecutors said they planned to show at trial that Slatten fired the first shots at Nisur Square, and that he had harboured "deep hostility" to Iraqi civilians.

He told people he wanted to "kill as many Iraqis as he could as 'payback for 9/11'," and boasted of killing an old Iraqi woman who had a knife in her hand because she had been chopping vegetables, prosecutors said.

Blackwater is now known as Academi and is based in McLean, Virginia.