A statement from U.S.-led coalition forces in Baghdad confirmed the shooting, which wounded freed journalist Giuliana Sgrena, but said the car had approached a check point at a high rate of speed and had ignored warnings to stop. The vehicle was en route to the airport after Sgrena's release from her captors in Iraq.
CBS News National Security Correspondent David Martin reports Pentagon officials have confirmed that it was an apparent mistake.
Berlusconi said U.S. troops took Sgrena, 56, to a U.S. military hospital, where she had a minor operation on her left shoulder to remove a piece of shrapnel.
The two other intelligence agents in the car were also wounded, the premier said. The U.S. military, however, said one person was killed and two were wounded, presumably including the journalist. The Apcom news agency said one was in serious condition and is believed to have suffered a lung injury.
Sgrena was abducted Feb. 4 by gunmen who blocked her car outside Baghdad University.
Both Berlusconi and Gabriele Polo, the editor of Sgrena's newspaper, Il Manifesto, said the secret service agent was killed when he threw himself over the freed hostage to protect her from U.S. fire.
Berlusconi identified the agent killed as Italian intelligence officer Nicola Calipari, and said he had been at the forefront of negotiations with the kidnappers. The premier said Calipari had been involved in the release of Italian hostages in the past.
The Italian premier, an ally of the United States who has kept troops in Iraq despite public opposition at home, said he has asked the U.S. ambassador for an explanation.
"Given that the fire came from an American source, I called in the American ambassador," Berlusconi told reporters. "I believe we must have an explanation for such a serious incident, for which someone must take the responsibility."
In other recent developments:
Mel Sembler, the U.S. ambassador in Italy, was expected to see Berlusconi on Friday evening, the embassy in Rome said. It had no further comment.
The U.S. 3rd Infantry Division, which controls Baghdad, said in an announcement that "U.S. soldiers killed one civilian and wounded two others when their vehicle traveling at high speeds refused to stop at a check point here today."
It said a U.S. patrol "attempted to warn the driver to stop by hand and arm signals, flashing white lights, and firing warning shots in front of the car," the military said in a statement. "When the driver didn't stop, the soldiers shot into the engine block which stopped the vehicle, killing one and wounding two others."
The shooting occured at 8:55 p.m. (1755 GMT), an earliuer statement said, and "the recently freed Italian journalist Giuliana Sgrena was an occupant in the vehicle and was apparently injured," and was being treated by coalition force medical personnel.
Berlusconi said he had been celebrating Sgrena's release with the Il Manifesto editor, and with Sgrena's partner, Pier Scolari, when he took a phone call from an agent who informed them of the shooting.
"It's a shame that the joy we all felt was turned into tragedy," Berlusconi said.
The shooting came as a blow to Berlusconi, a close ally of U.S. President George W. Bush, who has deployed 3,000 troops in Iraq despite opposition in Italy. The incident was certain to set off new protests against the Italian military presence. Sgrena's left-wing newspaper was a loud opponent of the war.
Last month, Sgrena was shown in a video pleading for her life and demanding that all foreign troops — including Italian forces — leave Iraq.
On Feb. 19, tens of thousands of demonstrators marched through Rome waving rainbow peace flags to press for Sgrena's release. Il Manifesto and Scolari organized the march. Scolari actively has highlighted Sgrena's pacifist conviction in hopes of aiding her release.
More than 190 foreigners have been abducted in Iraq in the past year, and more than 30 of the hostages were killed.
Another European reporter, Florence Aubenas, a veteran war correspondent for France's leftist daily Liberation, is still being held in Iraq. Aubenas and her interpreter, Hussein Hanoun al-Saadi, disappeared nearly two months ago.