America's electronic spying chief says the massive surveillance programmes revealed by an ex-intelligence worker have disrupted dozens of terror plots.
National Security Agency (NSA) director General Keith Alexander defended the internet and telephone data snooping programmes, known as Prism, before a Senate Committee hearing.
"Great harm has already been done by opening this up and the consequence, I believe, is our security has been jeopardised," he said.
The BBC reports US Secretary of State John Kerry said they showed a "delicate but vital balance" between privacy and security.
The programmes were revealed in newspaper accounts last week.
General Alexander also denied a claim it was possible for the whistleblower, Edward Snowden, to wiretap any American from his laptop.
Mr Snowden says he could even wire tap the president when he worked as a contractor at the agency.
Meanwhile, Mr Snowden, 29, has pledged to fight extradition to the US.
He fled his home in Hawaii for Hong Kong shortly before reports of the top secret programmes were published by the Guardian and Washington Post newspapers last week.
US officials have confirmed the programmes exist, with President Barack Obama saying they were closely overseen by Congress and the courts, reports the BBC.
