Silvio Berlusconi, the Italian Prime Minister, said today that he was insisting on "maximum collaboration" from the United States with investigations into the death of an Italian secret service agent killed by American forces in Baghdad last week.

In his first major address since Friday’s shooting strained relations between Washington and one of its biggest allies, Signor Berlusconi told Italy’s Senate: "We have the duty to demand the maximum collaboration to obtain the truth and determine who is responsible."

Signor Berlusconi said that Nicola Calipari, who died in a hail of bullets as he escorted a freed Italian hostage to Baghdad airport, had full US military authorisation for his operation to win the hostage's release.

The idea that Calipari was killed by friendly fire is "painful" to accept, Signor Berlusconi said, but he reassured lawmakers: "The United States has no intention of evading the truth."

"I’m sure that in a very short time every aspect of this will be clarified," he said.

Signor Berlusconi's ten-minute address made no mention of ransom payments to win the release of hostage Giuliana Sgrena, a journalist with the Communist newspaper Il Manifesto. Italian newspapers have reported that a ransom of $6 million (£3.1 million) was paid to win the reporter's freedom.

President Bush sent a letter to President Ciampi, his Italian counterpart, renewing his promise for a swift and thorough investigation into the killing. Italian officials will be involved in the inquiry.

Richard Owen, Times correspondent in Rome, said Signor Berlusconi was clearly aware that opinion polls have consistently shown a majority of Italians are opposed to the Italian deployment in Iraq. The Government sent 3,000 troops to southern Iraq after the ouster of Saddam Hussein.

"He's obviously trying to ensure that he does not inflame feelings against Italian involvement in the US-led coalition, while ensuring that there are clear answers about Calipari's death," Owen said.

And he said that the Prime Minister also pointed out to senators that while Italians have been kidnapped in Iraq, they have not been targeted any more than nationals of countries that are not in the coalition.

Italy has called the shooting an "accident," echoing the White House’s characterisation of the death, but officials have disputed the US version of events on the road to Baghdad. Signor Berlusconi said today the car that the dead agent was travelling in stopped immediately when a light was flashed at it.

"The case of friendly fire is certainly the most painful to bear. It feels like an injustice beyond any

sentiment. It’s something unreasonable," Signor Berlusconi said.

"When Italian citizens have been victims of kidnappings, the government has always acted by following two directives: It has always rejected political blackmail, while at the same time activating all the political, diplomatic and intelligence channels to obtain the release of our nationals," he said.

In his letter to Signor Ciampi, Mr Bush called the shooting a "terrible tragedy" and expressed his solidarity, the Italian President's office said in a statement.

"In his letter, President Bush assures President Ciampi that the United States will move toward a swift and exhaustive joint investigation; between Rome and Washington," the statement added.

The US-led coalition in Iraq announced today that it was ordering an investigation into the shooting, to be led by a US brigadier with Italian officials’ participation.

Signor Berlusconi's address to the Senate came a day after Gianfranco Fini, the Foreign Minister, went before Parliament to dismiss the idea that the shooting was the result of an ambush, but also to dispute that the vehicle was speeding up and was ordered to stop before the shooting, as Americans have indicated.

The possibility of an ambush had been raised by Signora Sgrena, who in some of her first comments since being released contended that the United States disapproved of Italy’s method of negotiations with kidnappers.

Signora Sgrena today tried to distance herself from the ambush hypothesis. "I never said that they wanted to kill me, but that the mechanics of what happened are that of an ambush," she told state television.

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