General Keith Alexander, the director of the U.S. National Security Agency, said on Tuesday (June 18) that the NSA's data gathering programs had prevented potential terrorist attacks more than 50 times since Sept. 11, 2001.

"In recent years these programs, together with other intelligence, have protected the U.S. and our allies from terrorist threats across the globe to include helping prevent ... potential terrorist events over 50 times since 9/11," he said in testimony to the U.S. House of Representatives intelligence committee.

Alexander and other U.S intelligence officials have made several trips to Capitol Hill recently to testify to congressional committees on the value of the sensitive NSA surveillance programmes which have fallen under criticism following the leaks of classified information by Edward Snowden, a former NSA contractor.

The deputy director of the U.S. Federal Bureau of Investigation outlined details of several plots which were foiled with the help of information collected under the surveillance program, including one planned attack on the New York Stock Exchange.

While monitoring a known extremist in Yemen, intelligence agents "were able to detect a nascent plot to bomb the New York Stock Exchange," FBI Deputy Director Sean Joyce told the committee.

Joyce also said the surveillance programmes were instrumental in identifying Pakistani-American David Headley as a co-conspirator in the bombing of a hotels in Mumbai and other coordinated attacks in which 166 people were killed, and in capturing Afghan-American Najibullah Zazi before he carried out a plan to bomb the New York subway system in 2009.

Presented by Adam Justice