Keith Alexander, NSA head, says surveillance stopped 50 terror plots in 20 nations (VIDEO)

The head of the US National Security Agency (NSA), Keith Alexander, on Tuesday defended his agency's surveillance efforts before the House Permanent Select Committee on Intelligence. 

Alexander claimed agency efforts helped put a stop to as many as 50 terrorist plots in 20 countries, describing more than 10 of those as containing a "domestic nexus," said the Guardian. 

The Federal Bureau of Investigation's deputy director, Sean Joyce, another witness, went into more detail. He told lawmakers the NSA's controversial PRISM program helped thwart a bomb attack on the New York Stock Exchange as well as a planned 2009 attack on the New York City subway system, according to Politico, while the Guardian said only one of the plots described by Joyce reached beyond a "nascent" stage.

The Washington Post recently revealed the details of the NSA program, code-named PRISM, as a secret data-mining program that reportedly gleaned information from major technology companies, such as Apple, Facebook, and Google. The news outraged civil rights and privacy advocates. 

More from GlobalPost: PRISM: Apple, Facebook, Microsoft … Who released which data?

The Post's source was whistleblower Edward Snowden, a former contractor with access to the NSA's program, who went public about some of the agency's surveillance and hacking practices.

Alexander told lawmakers on Tuesday that Snowden's leak caused "irreversible and significant damage," according to CBS News.

He also defended the agency's practices. "I would much rather be here today debating this point than trying to explain how we failed to prevent another 9/11," he told lawmakers, said The Guardian. 

Tuesday's hearing focused on national security issues, according to CNN, with lawmakers particularly concerned with how the agency protects Americans from attacks on the homeland. 

Watch Alexander on Snowden here: