Italian Defence Minister Antonio Martino says Saddam Hussein loyalists and al-Qaeda members are to blame for the attack on Italian troops in Iraq.
Mr Martino says there is a 'fairly reliable' al-Qaeda link
Eighteen Italians died in the suicide bomb attack in Nasiriya which killed at least nine others, injuring many more.
Italian Prime Minister Silvio Berlusconi has said his country would not be deterred by the incident.
France, meanwhile, has urged Washington to hand over power as quickly as
possible to the Iraqi people.
The French foreign minister, Dominique de Villepin, said the international community could wait no longer - more time meant more people dead, he said.
Earlier on Thursday, Japan said it would postpone sending its troops to Iraq until next year because of the security situation.
Italian officials confirmed on Wednesday evening the deaths of 16 Italian military and police personnel, two Italian civilians and nine Iraqis.
A further 20 Italians and 59 Iraqis were wounded in the attack in which a suicide squad driving a petrol tanker rammed Italian police headquarters in Nasiriya, a Shia Muslim town in the south of the country.
It was the single biggest loss of life for a member of the US-led coalition since the start of the war in Iraq in March.
Concern over security in the town has also led to a Portuguese contingent, the first to be deployed in Iraq, being sent to Basra instead.
After visiting the injured in hospital and the scene of the blast, Mr Martino said there was evidence to suggest that al-Qaeda and Saddam Hussein loyalists had organised and executed the attack.
"I went to visit Ground Zero in New York and I had the same impression," he said.
"The enemy we have to deal with is the same one: Global terrorism, which is striking wherever possible."
Mr Martino said a truck, followed by an armoured car, approached the compound at high speed before gunmen inside one of the vehicles opened fire at Italian troops guarding the entrance.
The guards returned fire, but the vehicle ploughed through the gate, and then exploded, he added.
The BBC's Peter Greste, in Baghdad, said the bombing backs up a leaked CIA report that says the Iraqi insurgency is both better organised and supported than was previously thought, and that American military tactics are making things worse.
In Italy, Prime Minister Berlusconi's words were echoed by the Italian President, Carlo Azeglio Ciampi, who said the troops in Iraq were there "with a mandate and the will of parliament".
"All of Italy is behind them," he said, before leaving for the United States where he will meet President Bush and United Nations Secretary General Kofi Annan.
However, the leftist opposition bitterly condemned Mr Berlusconi's decision to contribute troops to the US-led occupation.
"They were sent to an Iraq in flames because the government wanted to do a favour for the Bush administration without taking risks into consideration," said Pietro Folena of the main opposition party, the Democrats of the Left.
"Now the Italian soldiers must come home. It is the only right thing to do at this moment."