Karzai, whose own shaky grip on Afghanistan is under threat from resurgent Taliban rebels, also urged another key contributor to the NATO security force, Canada, to stay the course.
"My message to the countries helping us in Afghanistan, to Canada, to Italy, is that the Afghan people, the Canadian people and the Italian people are in the same fight, a fight for the security of our lives today and tomorrow," Karzai told reporters after meeting NATO's Secretary-General, Jaap de Hoop Scheffer in Kabul.
Italy's Prime Minister Romano Prodi stepped down on Wednesday after his government lost a parliamentary vote over its foreign policy programme, including plans to keep Italian troops in Afghanistan. Most of the 2,000 forces are deployed in the relatively quiet west of the country.
De Hoop Scheffer, joined by the alliance's top military commander Gen John Craddock, said that he remained confident Italy "will not forsake and will not leave Afghanistan".
"Afghanistan ... is the frontline in the fight against those people who want to destroy the fabric of our societies," he said. "If we do not succeed in Afghanistan, I am quite sure that the spoilers will come to us, to Netherlands, to Belgium, to United Kingdom."