The Italian government has asked Washington for an explanation of the alleged abduction by CIA agents of an Egyptian Islamic cleric in 2003.
The imam was allegedly flown out of Italy from a US military base
Officials in Rome say they had no prior knowledge of the operation, in which Osama Mustafa Hassan was reportedly flown to Egypt for interrogation.
But reports from the US say CIA officials briefed their Italian counterparts about the Milan operation.
Last week Italy issued arrest warrants for 13 people they said were agents.
Mr Hassan, also known as Abu Omar, was already being investigated in Italy as part of a terrorism inquiry.
Italian prosecutors believe the operation was part of a controversial US anti-terror policy known as "extraordinary rendition".
The policy involves seizing suspects and taking them to third countries without court approval.
Italian officials 'informed'
Carlo Giovanardi, Italy's minister for relations with parliament, was quoted by AP news agency as telling senators that neither the Italian government nor its intelligence services knew about the operation.
It was "not even possible" that Rome ever sanctioned it, he said.
But the Washington Post quoted unnamed CIA veterans as saying that the CIA station chief in Rome had informed Italian officials in advance.
They said that it was agreed that if the operation became public neither side would confirm its involvement.
No arrests have been made in the case since the warrants were issued. None of the suspects is currently believed to be in Italy.
'Cry for help'
An Egyptian woman said she had witnessed the abduction of Mr Hassan on 17 February 2003, while he was walking from his house to the mosque where he preached.
She told police he was stopped by two men dressed as police officers, and cried for help in Arabic as he was bundled into a white van.
According to Italian daily Corriere della Sera, Mr Hassan was then driven to the US base at Aviano north of Venice and transferred to another base in Germany, before eventually being taken to Egypt.
The 42-year-old imam called his family in Italy after being released last year, and said he had been tortured with electric shocks during his detention.
Mr Hassan is believed to have arrived in Italy in 1997, where he was granted refugee status.
Italian investigators say his abduction hampered an ongoing investigation into alleged terrorist links.
They managed to track down the 13 suspected agents through the Italian mobiles they used during the operation.
The suspects are said to have used US passports to check in at several top-range hotels in Milan.