However Mr Berlusconi said the incident should not mar Italy’s relationship with the US.
“The absence of deliberateness doesn’t indeed rule out blame,” he said in the Italian parliament.
The US report said its troops deserved no blame in the March 4 death of Mr Calipari, who was on a mission to free kidnapped Italian journalist Guiliana Sgrena when he was shot dead at a US military checkpoint near Baghdad airport.
Italy and the US held a joint inquiry into the incident, however issued conflicting reports.
The prime minister said irregularities committed by the US patrol, whose “mobile checkpoint was not announced”, and who “did not have precise instructions”.
But he said “the friendly ties between Italy and the United States cannot be called into question.”
“And I want to dispel possible confusion: there is no link between the killing of Calipari and our country’s mission in Iraq,” he added.
His comments come a day after a “long and cordial” telephone conversation with US President George W Bush about the incident.
The US report claimed the vehicle was travelling at about 50mph and did not stop at the checkpoint when ordered.
However the Italian probe said stress and inexperience on the soldiers’ part played a role, and said the car had been travelling at a much slower speed.
The US claimed it had not been informed of the plan to rescue Ms Sgrena, however Italy disputed this.
Italians have largely opposed the war in Iraq, and Mr Berlusconi has faced mounting pressure to return home Italy’s 3,000-strong troop deployment.
“We must insist in our commitment and assist the forces of a free and democratic new Iraq,” said Mr Berlusconi.