Information collected by once-secret US surveillance programmes disrupted dozens of terrorist attacks, the director of the National Security Agency has told Congress.
The speech came as the man who leaked documents to expose the programmes declared from Hong Kong: "I am not here to hide from justice".
NSA director, Army General Keith Alexander, was set to address the Senate Intelligence Committee in closed session later. On Wednesday he insisted the public needs to know more about how the top-secret programmes operate amid increasing unease about rampant government snooping and fears that citizens' civil liberties are being trampled.
"I do think it's important that we get this right, and I want the American people to know that we're trying to be transparent here, protect civil liberties and privacy but also the security of this country," he told a Senate panel.
Half a world away, Edward Snowden, the former contractor who fled to Hong Kong and leaked details of the programs, said he would fight any US attempts to extradite him.
US law enforcement officials are building a case against him but have yet to bring charges.
"I am not here to hide from justice - I am here to reveal criminality," Mr Snowden said in an interview with the local South China Morning Post.
Gen Alexander described the steps the government takes once it suspects a terrorist organisation is about to act - all within the laws approved by Congress and under stringent oversight from the courts.
He said the programmes led to "disrupting or contributing to the disruption of terrorist attacks" but did not give details. Gen Alexander warned that revelations about the secret programmes have eroded agency capabilities and, as a result, the US and its allies will not be as safe as they were two weeks ago, adding: "Some of these are still going to be classified and should be, because if we tell the terrorists every way that we're going to track them, they will get through and Americans will die."
Gen Alexander said he was seriously concerned that Mr Snowden, a former employee with Booz Allen Hamilton, had access to key parts of the NSA network, a development that demands a closer examination of how well the agency oversees contract employees.