US Secretary of State John Kerry has conceded that some US data surveillance has gone "too far."
In remarks via teleconference to an "open government" summit in London on Thursday, Kerry defended espionage as necessary in the fight against terrorism but conceded restraint was necessary.
He indicated that the revelations of widespread snooping by the National Security Agency (NSA) through leaks by former government contractor Edward Snowden caught everyone by surprise.
"The president and I and others in government have actually learned of some things that had been happening, in many ways, on an automatic pilot because the technology is there," Kerry said.
He insisted that surveillance had produced information that had stopped airplanes "from going down, buildings from being blown up, and people from being assassinated."
Kerry also said that President Barack Obama was "determined" to do a thorough review of surveillance "in order that nobody will have the sense of abuse."
"In some cases, I acknowledge to you, as has the president, that some of these actions have reached too far, and we are going to make sure that does not happen in the future," Kerry said.
He disputed news reports that 70 million people were listened to.
"No, they weren't. It didn't happen. There's an enormous amount of exaggeration in this reporting from some reporters out there," he said.
Kerry also emphasised that the United States and others were working together to collect data, an apparent reference to countries with which top US intelligence officials say Washington was exchanging data.