Iraq has criticised a US judge's decision to dismiss charges against private security guards accused of killing Iraqi civilians.

The five guards from the company Blackwater Security had been charged with killing 14 unarmed Iraqi civilians during an attack at a busy Baghdad roundabout in 2007.

But now a US judge has dismissed manslaughter charges against them because of mistakes by the prosecution.

Iraq's Human Rights Minister says she is "astonished" by the decision.

An Iraqi Government spokesman also says that Iraq will act "forcefully and decisively to prosecute" the Blackwater guards.

"I was astonished by this decision," Human Rights Minister Wejdan Mikhail said.

"There was so much work done to prosecute these people and to take this case into court and I don't understand why the judge took this decision."

US federal judge Ricardo Urbina on Thursday dismissed the charges against the five, saying prosecutors violated their rights by using incriminating statements they had made under immunity during a State Department probe to build their case.

The case was among the most sensational that sought to hold Blackwater employees accountable for what was seen as a culture of lawlessness and a lack of accountability as it carried out its duties in Iraq.

The guards, who had been part of a convoy of armoured vehicles, had been charged with killing 14 unarmed Iraqi civilians and wounding 18 others during an unprovoked attack at a busy Baghdad roundabout using guns and grenades.

"One of them has said what happened in Nisur Square, how they killed innocent Iraqi people that were just in their cars without any weapons. I am very astonished and I am waiting for the US embassy to give me the judge's decision (in full)," the Iraqi human rights minister said.

"What happened was very bad, because so many innocent Iraqi people - young, students - were shot by someone who liked to shoot unarmed people."

Ms Mikhail added that she had requested a meeting with US embassy officials in Baghdad. The embassy did not immediately confirm that the meeting would take place or, if it did, who it would involve.

Government spokesman Ali al-Dabbagh said Iraq would "act forcefully and decisively to prosecute the Blackwater criminals".

He added that an Iraqi investigation had shown that the five guards were unquestionably responsible for the deaths of the civilians.

According to Iraq, 17 civilians were killed in the shooting, but the guards were charged with 14 deaths.

"Inquiries carried out by the Iraqi government clearly confirm that the Blackwater guards committed a crime and used weapons when there was no threat necessitating the use of force," Mr Dabbagh said.

Foreign security teams in Iraq long operated in a legal grey area, but under a military accord signed with Washington last November, Baghdad won a concession to lift the immunity to prosecution previously extended to US security contractors.

Ordinary Iraqis expressed anger at Judge Urbina's decision to dismiss the charges.

"Dropping the charges against those guards disrespects the lives of the innocents who were killed," said Abu Uday, a university professor who did not want to give his given name.

"The rights of the families of the victims must be guaranteed, those who lost their sons because of the rashness of guards who tried to show off their abilities with no respect for people's lives."

The top US commander in Iraq, General Ray Odierno, said he feared a backlash against private security firms working here.

"We all know that it was not US soldiers, sailors or marines who did this, it was a private security company," General Odierno told a press conference.

"What I worry about is there will be backlash against private security companies that continue to operate (in Iraq). I wouldn't like to see that."

Blackwater ended its operations in Iraq in May, after the US State Department refused to renew annual contracts for the company.

The firm renamed itself Xe after the Iraqi government banned it last January over the killings.

Headquartered in North Carolina, Blackwater was one of the largest security firms operating in Iraq with about 1,000 staff, and had been employed to protect US government personnel since the 2003 invasion.

- ABC/AFP