The prime minister told reporters after an EU summit in Brussels that he "did not know" if Italy has been targetted by the agency amid reports it had spied on 35 heads of State and government, but that he was "waiting for clarification".
A report in the Italian weekly, L'Espresso, said that Italy had been monitored by both Britain and the US.
Letta quizzed US Secretary of State John Kerry about the matter during talks in Rome on Wednesday. Kerry is reported to have said that he would look into the allegations.
Ahead of the EU summit on Friday, Letta said: "Obviously, all checks should be done, but we want the whole truth. It's not acceptable or conceivable that there are activities of this kind."
The report in L'Espresso said the espionage concentrated on three underwater fibre optic cables with terminals in Italy: the SeaMeWe3 and the SeaMeWe4 in Sicily and the Flag Europe Asia crossing the country.
"In this mass collection, our secret services had a role," the publication said, citing journalist Glenn Greenwald, who sparked global controversy with his revelations based on US leaker Edward Snowden.
Snowden's documents showed that the Italian security services had a "third-level agreement" with Britain's Government Communications Headquarters (GCHQ).
GCHQ's selection of Internet communications and phone calls included diplomatic, financial and military information and was then passed to the US National Security Agency (NSA), the report said.
The spying was on "European governments, including the Italian one", Greenwald was quoted as saying.
Among the latest revelations is that the US has been listening into German Chancellor Angela Merkel's mobile phone. Merkel told reporters on Friday that she has two mobile phones, and that the one the NSA listened in on was not used for important conversations.