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Iraq's prime minister Wednesday disputed Blackwater USA's version of a weekend shooting that left at least 11 people dead, saying he cannot tolerate "the killing of our citizens in cold blood."

More witnesses came forward saying they saw Blackwater security guards firing at civilians in the Mansour district of western Baghdad last Sunday.

Blackwater, which provides security for American diplomats and other civilian officials in Iraq, insisted its contractors were returning gunfire from armed insurgents.

U.S. and Iraqi officials announced they had formed a joint committee to investigate the killing and try to reconcile widely differing versions of the incident. Conflicting accounts were circulating among Iraqi officials themselves.

Land travel by U.S. diplomats and other civilian officials outside the fortified Green Zone remained suspended for a second day after Iraqi authorities ordered Blackwater to stop working as an investigation continues into the Sunday incident.

The Moyock, North Carolina-based firm is the main provider of bodyguards and armed escorts for American government civilian employees in Iraq.

U.S. Embassy spokeswoman Mirembe Nantongo refused to offer any version of what happened Sunday at busy Nisoor Square. She told reporters that the contractors involved in the incident were still in Iraq.

But Prime Minister Nouri al-Maliki spoke out sharply against Blackwater, saying the weekend shooting was "the seventh of its kind" involving the company "and these violations should be dealt with."

"We will not tolerate the killing of our citizens in cold blood," al-Maliki told reporters. "The work of this company has been stopped in order to know the reasons."

Blackwater spokeswoman Anne E. Tyrrell said in a statement late Monday that its employees acted "lawfully and appropriately" in response to an armed attack against a State Department convoy.

"The `civilians' reportedly fired upon by Blackwater professionals were in fact armed enemies and Blackwater personnel returned defensive fire," she said. "Blackwater regrets any loss of life but this convoy was violently attacked by armed insurgents, not civilians, and our people did their job to defend human life."

But al-Maliki said Blackwater's version "is not accurate" and that the killing had generated "widespread hatred and anger from the government and the people." He said Blackwater "should be held accountable for such a violation."

Iraqi officials offered several versions of the incident. One official said the Blackwater convoy got stuck in traffic and the guards began firing and throwing stun grenades to clear the vehicles.

Another official said gunmen in a passing car shot at the convoy and the Blackwater guards responded with heavy return fire, hitting civilians caught up in the incident. Others said a car bomb exploded and the guards opened fire.

All of the officials spoke on condition of anonymity because they were not supposed to release the information. Some accounts spoke of a child being killed, others that the infant was wounded.

Two Iraqi witnesses said they saw only Blackwater firing, although U.S. officials said Monday that one of the American vehicles had been disabled by gunfire.

"Several SUVs were passing from Nisoor Square when an explosion took place, I couldn't tell whether it was a roadside bomb or a car bomb," said Imad Mansour Abid, 35. "This was followed by heavy fire by guards of the security vehicles."

He said the shots were fired "at streets in the area where civilians and passers-by were moving. The firing lasted about 10 to 20 minutes."

Suhard Mirza, a hairdresser who works in the area, said she heard a "distant explosion" and raced outside to see what was happening.

"I saw four-wheel drive vehicles opening fire randomly on people and civilian cars in the area," she said. "After five minutes police and ambulances reached the area to evacuate casualties."

Eager to contain the crisis, the State Department said Wednesday that a joint U.S.-Iraqi commission will be formed to investigate the incident.

"We're in conversations with the Iraqis on how we can find some mechanisms for looking at this issue in a joint way," deputy spokesman Tom Casey told reporters in Washington.

"There have been a number of questions that have been raised and we want to make sure that both we and the Iraqis have a common set of facts that we're working from and also that we can hopefully come to some common conclusions on how to proceed," he said.

Also Wednesday, the U.S. military said an American soldier was killed the day before in an attack in southern Baghdad. Another soldier died Wednesday of non-battle related causes in Salahuddin province, the military said.

The Iraqi Cabinet decided Tuesday to review the status of all foreign security companies in the wake of the Mansour shooting. The Interior Ministry said Monday it had lifted Blackwater's license and ordered its 1,000 employees to leave the country.

The next day, Iraqi officials said Blackwater's operations were merely suspended until investigators can determine what happened Sunday.

Some Iraqi officials said privately it would be difficult to order Blackwater out of the country because the Americans rely so heavily on it for their security.

"It will be difficult for the Iraqi government to make them leave the country because they protect the embassy," said an aide to al-Maliki. "Maybe they will make a commitment that they study their moves" or agree to change the name of the company.

The aide spoke on condition of anonymity because the issue is so sensitive.

Iraqis have long resented the presence of thousands of armed foreign security guards, whose numbers swelled after violence escalated in late 2003 months after the fall of Saddam Hussein's regime.

With too few U.S. and coalition forces available to maintain order, governments and private companies hired thousands of security guards to protect their operations from Sunni and Shiite extremists and criminal gangs.

Blackwater, whose convoys of SUVs careen through the streets with weapons displayed, has been singled out for much of the criticism — in part because of its high profile operations.

"Blackwater has a reputation. If you want over-over-the-top, gun-toting security with high profile and all the bells and whistles, Blackwater are the people you are going to go with," said James Sammons, a former Australian Special Air Service commander who now works for British-based AKE Group that also provides security in Iraq.

He said any civilian killings by security contractors tarnish the reputations of all of them.

"We get lumped in with that and it makes the job harder for the rest of us," said Sammons, who is AKE's Asia-Pacific regional director, based in Sydney, Australia.