By Alissa J. Rubin and Andrew E. Kramer

Prime Minister Nuri Kamal al-Maliki said Sunday that the shooting of Iraqi civilians last week by Blackwater USA, a private American security company, amounted to a challenge to the nation's sovereignty, and he added that his government was working jointly with the United States to bring those responsible to justice.

In an interview with The Associated Press in his New York hotel suite before his appearance at the United Nations General Assembly, he said: "The Iraqi government is responsible for its citizens, and it cannot be accepted for a security company to carry out a killing. There are serious challenges to the sovereignty of Iraq." The A. P. said Mr. Maliki had used the Arabic word tajawiz, which can be translated either as affronts or challenges.

On Sept. 16, Blackwater security guards opened fire on civilians in Nisour Square in western Baghdad, killing at least 11 people and reminding Iraqis of the behavior of private Western security companies operating in the country. The episode was the seventh in which Iraqi authorities had cited Blackwater for the injurious behavior of its guards toward civilians. However, an Iraqi security official said the government was compelled to allow Blackwater to remain in operation in Iraq in spite of deep misgivings about the company's role here. Tahseen al-Sheikhly, a spokesman for the Iraqi security forces, said that immediately removing Blackwater's hundreds of armed guards could create a security vacuum in the capital, forcing American commanders to redeploy troops from elsewhere in the country. That could leave other volatile areas thinly patrolled. "If Blackwater left at this moment, it might leave a security gap because most of the embassies and most of the foreign organizations that are working in Iraq" rely on Blackwater, Mr. Sheikhly said at a news conference with a spokesman for the American military in Baghdad. "This will create a security imbalance." "That's why the Iraqi government preferred to be patient on activating this decision to stop them," he said. "But the government is still serious in finding certain rules" to govern private security contractors. "We would like to have some laws," he said.

Meanwhile, an American soldier was killed and another was wounded when a sophisticated roadside bomb, known as an explosively formed penetrator, detonated near their patrol during combat operations in East Baghdad on Saturday, the United States military said. A British soldier died Friday in Britain from wounds sustained in Iraq last week, the Defense Ministry said Saturday. Over all, however, the American commander in Baghdad said violence had continued to diminish in the capital. In a statement released Sunday, the commander, Maj. Gen. Joseph F. Fil Jr., said there had been a 70 percent decrease in the casualties caused by car bombs since an increase in the number of American troops in the city in mid-February. His statement also said that there had been a 125 percent increase in the rate at which car bombs were discovered by security forces before they were exploded by insurgents. Before February, just one-fifth of Baghdad's neighborhoods were free of organized insurgent activity. Now, more than half of Baghdad's neighborhoods have improved to the point that economic investment can begin, General Fil said.

The counterinsurgency doctrine embraced by the top United States' commander in Iraq, Gen. David H. Petraeus, is "clear, hold and build" — meaning that insurgents first need to be removed from neighborhoods, then the military needs to keep them from returning and then it is possible to start making investments and building up the area. However, in a signal that violence remains the norm here, 10 bodies were found in Baghdad on Sunday and there were clashes in Basra between two Shiite militias. There was also a reminder of the longstanding antagonism in Iraq toward the United Nations when a roadside bomb exploded Sunday near the organization's former headquarters in Baghdad, which is now deserted.

The United Nations secretary general, Ban Ki-moon, recently announced that the international organization would have a more robust presence in Iraq and would consider expanding its offices in the northern city of Erbil and perhaps would open an outpost in Basra, in the south of the country.

Ali Hamdani contributed reporting from Baghdad, and Iraqi employees of The New York Times from Basra and Hilla.