Keith Alexander was speaking to the US Senate committee.
The head of the US National Security Agency (NSA), Keith Alexander, has defended the surveillance programme, Prism, by saying that it helped to prevent dozens of possible attacks both in the country and abroad.
According to Reuters, Alexander told a US Senate committee that: "It’s dozens of terrorist events that these have helped prevent."
"Both here and abroad, in disrupting or contributing to the disruption of terrorist attacks," Alexander said.
Recently, US President Barack Obama has also defended the surveillance programme citing security concerns.
Obama said: "Nobody is listening to your telephone calls. That’s not what this programme’s about."
The European Union has raised concerns with the surveillance programme and said it would raise the matter with US Attorney General Eric Holder in Dublin on June 14.
Based on the data leaked by a former Central Intelligence Agency employee, Edward Snowden, the Guardian and the Washington Post have reported that the US authorities are having access emails, information of live chats of the citizens.
Alexander was quoted by Reuters as saying that: "Great harm has already been done by opening this up."
"There is no doubt in my mind that we will lose capabilities as a result of this and that not only the United States but those allies that we have helped will no longer be as safe as they were two weeks ago," Alexander said.