Aftermath of the bomb blast in Kabul
Six Italian soldiers have been killed in a bomb attack on a military convoy in the Afghan capital, Kabul.
Two military vehicles were reported to have been hit by a suicide car bomb. At least 10 civilians were also killed and dozens injured, officials said.
The Taliban have claimed responsibility for the attack in the city centre.
Following the blast, Italian PM Silvio Berlusconi said it would be best for international troops to leave Afghanistan "as soon as possible".
But he added it was a decision Italy would have to take in agreement with other Nato partners.
The latest deaths come as Afghan President Hamid Karzai reaffirmed his belief in last month's presidential election which is mired in allegations of widespread fraud.
Witnesses say an explosives-laden vehicle rammed into the Italian military convoy on Kabul's busy airport road.
"It was a suicide car bomb attack... It was against Italian forces," Kabul's chief of criminal investigations told the AFP news agency.
ATTACKS ON FOREIGN TROOPS
Sept 09: Six Italians killed, four wounded after attack on Kabul convoy
Aug 08: Ten French soldiers killed in ambush east of Kabul
July 08: Nine US soldiers killed in an attack in Kunar
June 08: Four British soldiers killed by a landmine in Helmand
July 07: Six Canadian soldiers and their Afghan translator killed by roadside bomb in Kandahar
June 05: Sixteen US special forces and crew killed when helicopter shot down in Kunar
At least four Italian soldiers are also said to have been seriously wounded.
Eyewitnesses said the explosion shook buildings and that a plume of black smoke hung over the area where it occurred.
Student Jamal Nasir was in his car when the bomber struck and described an almost deafening blast.
"I could see thick black smoke behind us... There were screams and car horns. People were running over each other and in the panic cars were driving on pedestrian lanes," he told the BBC.
The blast caused considerable destruction in the immediate vicinity, with a number of shops badly damaged.
Witnesses described blood-stained roads and twisted metal littering the area.
Television footage of the blast site showed the Italian flag on Isaf armoured vehicles, one of which was destroyed.
Mr Berlusconi spoke to reporters about the attack as he arrived for an EU summit in Brussels.
"This is an unhappy day for Italy," he said. "We are all convinced that we have to get out of Afghanistan as soon as possible."
But he quickly added: "This is something that we cannot decide alone, because otherwise we would affect the confidence of other countries present there."
There have been several bomb attacks in Kabul this year.
At least 10 Afghan civilians are said to have been killed.
Shortly before nationwide elections in August, Nato's Kabul headquarters was hit by a suicide car bomb attack which killed at least 10 people.
But this is thought to be one of the deadliest attacks on foreign troops in recent months.
On Wednesday, three US troops were killed by a roadside bomb in southern Afghanistan.
There are about 3,200 Italian troops in Afghanistan, mostly in the west of the country.
Around 20 Italian soldiers have died in Afghanistan.
US and Nato-led forces across Afghanistan are battling a fierce Taliban insurgency.
In recent months the US has poured more forces into the Taliban's heartland in the south of the country.
And on Wednesday, the top US military officer, chairman of the Joint Chiefs of Staff Adm Mike Mullen, told the US Senate that yet more troops might be required in Afghanistan.
The year 2009 has been the deadliest for foreign troops in Afghanistan since the fall of the Taliban in 2001.