"All the elements lead us to believe he has been kidnapped and that is how we are treating the case," a Foreign Ministry spokesman said.
Torsello's abduction had already been reported by media and aid workers in Afghanistan, but Italian authorities had not commented.
He was seized by five gunmen on the highway from the capital of Helmand province to neighbouring Kandahar province, Afghanistan's independent Pajhwok news agency quoted travelling companion Gholam Mohammad as saying on Saturday.
Pajhwok said its call to Torsello's mobile phone was answered by a man saying: "We are the Taliban and we have abducted the foreigner on charges of spying."
But a Taliban spokesman told Reuters the group was not involved in any abduction, blaming criminals instead.
The latest abduction came as two more NATO soldiers, both Canadian, died in combat in the south of Afghanistan on Saturday in the bloodiest year since a US-led coalition ousted the Taliban in 2001.
An Italian online newspaper, PeaceReporter, which specialises in reports from conflict zones, said Torsello had confirmed by phone he had been kidnapped, but not by whom.
PeaceReporter said he had spoken briefly to the security chief at a hospital run by the Italian relief organisation Emergency in the Helmand capital of Lashkar Gah.
Torsello, a Muslim based in London, said he did not know where he was being held. He said he had been kidnapped on Thursday from a public bus, according to PeaceReporter.
Helmand and Kandahar are Afghanistan's most dangerous provinces and have been the scene of heavy fighting in the past few months between Taliban guerrillas and NATO forces.

Copyright Reuters, 2006