Two phone and internet surveillance programs run by the U.S. National Security Agency ( NSA) have helped disrupt more than 50 potential terrorist attacks against the United States and its allies, said the agency's chief on Tuesday, in another attempt to defend the programs that have been under fire.

U.S. Army Gen. Keith Alexander, commander of the U.S. Cyber Command, director of National Security Agency (NSA), testifies before a Senate Appropriations Committee hearing in Washington D.C. on June 12, 2013. [Fang Zhe/Xinhua]

Army General Keith Alexander, the Director of the National Security Agency and Commander of U.S. Cyber Command, defended the recently revealed surveillance programs before members of the House of Representatives Intelligence Committee.

"Let me start by saying that I would much rather be here today debating this point than trying to explain how we failed to prevent another 9/11," said Alexander.

He stressed that these two programs have held great value to " our national security and that of our allies" in combating terrorism.

"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 the terrorist -- the potential terrorist events over 50 times since 9/ 11," he claimed.

The plots included a previously undisclosed plan to blow up the New York Stock Exchange, Alexander added.

He noted that the September 11 attacks in 2001 occurred in part because of "a failure on the part of our government to connect those dots," some of which were already in the country.

During Tuesday's hearing, Alexander and officials from the FBI and the Office of the Director of National Intelligence provided details of a plot to blow up the Stock Exchange in New York.

The four-star general also told members that agency officials planned to provide them with an unclassified summary of the foiled terror plots to Congress by Wednesday.

The Congress panel hearing was the first of its kind dedicated to the recent disclosures of the phone and internet surveillance programs by the National Security Agency (NSA).

Two classified programs, one collecting U.S. phone records and the other mining internet data, were revealed last week after leaks from 29-year-old defense contractor Edward Snowden.

"Over the past few weeks, unauthorized disclosures of classified information have resulted in considerable debate in the press about these two programs," said Alexander.

In a Senate panel hearing last Wednesday, Alexander vowed to release more specific information about the number of attacks foiled by the two programs. He also said the agency would need to look again at the access of its members and its hiring processes.

U.S. President Barack Obama and other officials of the U.S. intelligence community have argued that the congressional, executive and judicial levels provided oversight over these surveillance programs. Obama also insisted that the tracking of internet activity had not applied to U.S. citizens or people living in the country.

According to the Guardian and the Washington Post reports last Thursday, the NSA and the FBI had been secretly tapping directly into the central servers of nine U.S. internet companies, extracting audio, video, photographs, e-mails, documents and connection logs that enable analysts to track a person's movements and contacts over time.?