WASHINGTON (AFP) - US surveillance efforts foiled over 50 potential terrorist events since the 2001 attacks, the head of a US spy agency told lawmakers on Tuesday in defending the programmes as vital to national security.
"These programmes are immensely valuable for protecting our nation and securing the security of our allies," said General Keith Alexander, director of the secretive National Security Agency (NSA), in the latest bid by the government to deflect criticism of the telephone and Internet snooping programs that came to light last week.
"In recent years the information gathered from these programmes provided the US government with critical leads to help prevent over 50 potential terrorist events in more than 20 countries around the world."
Gen Alexander said that "at least 10 of these events included homeland-based threats".
The NSA chief told the House Intelligence Committee that he would be providing lawmakers with information on the incidents foiled since the Sept 11, 2001 attacks.
"It's over 50 cases," he said, adding that the exact number was still being reviewed.
He said that details of four incidents were being made public, including plots to bomb the New York Stock Exchange and the subway system in New York.
Gen Alexander also said the programmes had a sound legal foundation with oversight by courts and Congress.
"I believe we have achieved the security and relative safety in a way that does not compromise the privacy and civil liberties of our citizens," he said.
Gen Alexander and other officials appeared at the hearing to discuss the firestorm after the release of news reports disclosing the vast data gathering of data of phone records and monitoring of Internet communications.
The reports said the so-called Prism programme directly obtained data from major US Internet firms including Google, Yahoo! and Facebook.
But Gen Alexander said that under this programme, "the US government does not unilaterally obtain information from the servers of US companies.
"Rather, the US companies are compelled to provide these records by US law, using methods that are in strict compliance with that law," he said.