The director of the National Security Agency has said that recently disclosed top-secret US surveillance programmes have helped to prevent "dozens" of potential attacks in the country.
NSA chief General Keith Alexander made the comments at a Senate hearing on Wednesday when asked if the intelligence community could estimate how much the agency's broad monitoring of phone call and internet data had helped preventing attacks.
Democratic Senator Patrick Leahy of Vermont said some details of two cases have been declassified. He pressed Alexander for numbers on other incidents.
Leahy said the intelligence community believes that these surveillance programmes are critical, but added that he thinks they should not be made permanent and that Congress should be able to review and debate them periodically.
Alexander also said his top-secret unit operates under "strict guidelines" and "rigorous oversight".
We operate in a way that ensures we keep the trust of the American people because that trust is a sacred requirement
"We operate in a way that ensures we keep the trust of the American people because that trust is a sacred requirement," he said.
"We do not see a trade-off between security and liberty."
Alexander's remarks did not specifically address revelations about the Prism programme, described as a wide-ranging system of surveillance in coordination with major internet companies, but it was his first appearance before Congress since the explosive revelations by the Guardian newspaper last week.
Alexander said these and other government agencies are "deeply committed to compliance with the law and the protection of privacy rights," even though details of the operations are classified.
"Given the nature of our work, of course, few outside of our executive, legislative and judicial branch oversight bodies can know the details of what we do or see that we operate every day under strict guidelines and accountability within one of the most rigorous oversight regimes in the US government," he said.
Meanwhile, Edward Snowden, the former CIA worker who leaked the details of the surveillance programmes, said in an interview with to the South China Morning Post, that he will ask the courts and people of Hong Kong to decide on his fate.
The newspaper said he was in Hong Kong but at a secret location. It was the first time Snowden had emerged from hiding since his explosive revelations in the Guardian last week.
Barrister Kevin Egan, who has previously dealt with extradition cases in the city, however said Snowden's best option may be to get out quickly.
"If I was him, I'd be getting out of here and heading to a sympathetic jurisdiction as fast as possible and certainly before the United States issues a request for his extradition," Egan told Reuters news agency.
"The attitude of the judiciary here seems to be if Uncle Sam wants you, Uncle Sam will get you."