US intelligence leaker Edward Snowden effectively stole the "keys to the kingdom" when he swiped more than 1.5 million top secret files, a senior National Security Agency official said in an interview.
Rick Ledgett, who heads the NSA taskforce in charge of assessing the impact of Snowden's leaks, told 60 Minutes that the contractor possessed a "roadmap" of the US intelligence community's strengths and weaknesses.
NSA chief General Keith Alexander meanwhile said on Sunday that suggestions the agency was routinely eavesdropping on the phone calls of Americans was false, insisting that less than 60 "US persons" were currently being targeted worldwide.
Ledgett said of particular concern was Snowden's theft of around 31,000 documents the NSA official described as an "exhaustive list of the requirements that have been levied against the National Security Agency."
"What that gives is, what topics we're interested in, where our gaps are," said Ledgett. "Additional information about US capabilities and US gaps is provided as part of that."
The information could potentially offer a rival nation a "roadmap of what we know, what we don't know, and give them - implicitly - a way to protect their information from the US intelligence community's view," the NSA official added.
"It is the keys to the kingdom."
Ledgett said he would be open to the possibility of an amnesty for Snowden, who remains exiled in Russia, if he agreed to stop further leaks of classified information.
"My personal view is, yes, it's worth having a conversation about" a possible deal, said Ledgett.
Snowden has been charged with espionage by US authorities for divulging reams of secret files.
The former NSA contractor has insisted he spilled secrets to spark public debate and expose the NSA's far-reaching surveillance.
But Alexander rejected the idea of any amnesty for Snowden.
"This is analogous to a hostage-taker taking 50 people hostage, shooting 10 and then say 'You give me full amnesty and I'll let the other 40 go'," Alexander told the US version of 60 Minutes, which airs on CBS.
Alexander also challenged the view that the NSA was engaged in widespread surveillance of Americans.
"NSA can only target the communications of a US person with a probable cause finding under specific court order," he said, referring to the secret Foreign Intelligence Surveillance Court.
"Today, we have less than 60 authorisations on specific persons to do that."
