Secret US surveillance has thwarted dozens of potential attacks, the head of the National Security Agency said as he defended his agency after leaks revealed hidden programs.
"It's classified, but it's dozens of terrorist events that these have helped prevent," NSA chief General Keith Alexander told lawmakers as he was questioned on the importance of internet and phone surveillance efforts.
Following an uproar over the disclosure about NSA-led programs to monitor internet and phone data, Alexander said, "I want this debate out there."
"I think what we are doing to protect Americans is the right thing," he said.
"Our agency takes great pride in protecting our nation and civil liberties ... this is what our nation expects our government to do."
Alexander said the attacks foiled included "dozens for both here and abroad, in disrupting or contributing to the disruption of terrorist attacks."
Appearing at a congressional hearing, Alexander said his agency was working to provide more data to lawmakers and the public. "I want the American people to know that we're being transparent in here," he said.
He maintained that the agency needs "reasonable articulable suspicion" before it gets a warrant to look more closely at data.
Alexander said that when the NSA seeks specific phone data, "that it is auditable by the (congressional) committees, by the courts, by the administration."
Asked whether NSA could determine what people are looking for on the search engine Google, he said: "Yes we could. You could get a court order... to do any kind of search in this area, you would need a court order."
Alexander said part of the mission of the NSA and the Cyber Command he heads is to protect so-called critical infrastructure, which includes power grids, water systems and computer networks which support them.
"The United States is already a target," he said.
"Networks and websites owned by Americans and located here have endured intentional, state-sponsored attacks, and some have incurred degradation and disruption because they happened to be along the route to another state's overseas targets," he said.
"Our critical infrastructure is thus doubly at risk. On a scale of one to 10, with 10 being strongly defended, our critical infrastructure's preparedness to withstand a destructive cyber attack is about a three based on my experience.