The head of the US National Security Agency has defended government's sweeping electronic surveillance programmes and said it had helped disrupt possible attacks more than 50 times since September 11, 2001.
Justifying the spying programmes that were disclosed by contractor Edward Snowden earlier this month, General Keith Alexander said on Tuesday he would give lawmakers classified details of all of the thwarted incidents within 24 hours.
He said the public disclosure of spying programmes caused "irreversible" damage to national security and helped America's enemies.
"I think it was irreversible and significant damage to this nation," General Keith Alexander said at a House of Representatives Intelligence Committee hearing.
Asked whether the disclosures by Snowden had helped America's enemies, Alexander replied: "I believe it has and I believe it will hurt us and our allies."
Alexander spoke at a rare open Capitol Hill hearing.
At the hearing, both the FBI and leaders of the House Intelligence Committee have come out in vigorous defence of the NSA programmes.
Deputy FBI director Sean Joyce said that the government's surveillance of telephone and Internet communications foiled plots including one to bomb the New York Stock Exchange.
Michigan Republican Representative Mike Rogers, who will preside over an open hearing of the intelligence panel later on Tuesday, said he expected NSA head to declassify additional information about the programme.
The Obama administration already has declassified data crediting the NSA programme with breaking up a planned attack on New York City's subway system.
On Monday, US President Barack Obama defended the NSA as legal and transparent in a lengthy TV interview.
"We're going to have to find ways where the public has an assurance that there are checks and balances in place ... that their phone calls aren't being listened into; their text messages aren't being monitored, their emails are not being read by some big brother somewhere," Obama said.
Meanwhile, Yahoo is the latest company to disclose how many requests for user data it has received from US government agencies, putting the number between 12,000 and 13,000 in the six months that ended on May 31.