RIO DE JANEIRO - Edward Snowden has highly sensitive documents on how the National Security Agency is structured and operates that could harm the US government, but has insisted that they not be made public, a journalist close to the NSA leaker said.

Glenn Greenwald, a columnist with The Guardian newspaper who first reported on the intelligence leaks, told The Associated Press that disclosure of the information in the documents ''would allow somebody who read them to know exactly how the NSA does what it does, which would in turn allow them to evade that surveillance or replicate it.''

He said the ''literally thousands of documents'' taken by Snowden constitute ''basically the instruction manual for how the NSA is built.''

''In order to take documents with him that proved that what he was saying was true he had to take ones that included very sensitive, detailed blueprints of how the NSA does what they do,'' the journalist said Sunday in a Rio de Janeiro hotel room. He said the interview was taking place about four hours after his last interaction with Snowden.

Greenwald said he believes the disclosure of the information in the documents would not prove harmful to Americans or their national security, but that Snowden has insisted they not be made public.

''I think it would be harmful to the US government, as they perceive their own interests, if the details of those programs were revealed,'' he said.

He has previously said the documents have been encrypted to help ensure their safekeeping.

Snowden emerged from weeks of hiding in a Moscow airport Friday, and said he was willing to meet President Vladimir Putin's condition that he stop leaking US secrets if it means Russia would give him asylum until he can move on to Latin America.

Greenwald told The AP that he deliberately avoids talking to Snowden about issues related to where the former analyst might seek asylum in order to avoid possible legal problems for himself.

Snowden is believed to be stuck in the transit area of Moscow's main international airport, where he arrived from Hong Kong on June 23. He's had offers of asylum from Venezuela, Nicaragua and Bolivia, but because his US passport has been revoked, the logistics of reaching whichever country he chooses are complicated.

Still, Greenwald said that Snowden remains ''calm and tranquil,'' despite his predicament.

''I haven't sensed an iota of remorse or regret or anxiety over the situation that he's in,'' said Greenwald, who has lived in Brazil for the past eight years. ''He's of course tense and focused on his security and his short-term well-being to the best extent that he can, but he's very resigned to the fact that things might go terribly wrong and he's at peace with that.''

Greenwald said he worried that interest in Snowden's personal saga had detracted from the impact of his revelations, adding that Snowden deliberately turned down nearly all requests for interviews to avoid the media spotlight.

Asked whether Snowden seemed worried about his personal safety, Greenwald responded, ''he's concerned.''

He said the US has shown it's ''willing to take even the most extreme steps if they think doing so is necessary to neutralize a national security threat.''

''He's aware of all those things, he's concerned about them but he's not going to be in any way paralyzed or constrained in what he thinks he can do as a result of that.''

Asked about a so-called dead man's pact, which Greenwald has said would allow several people to access Snowden's trove of documents were anything to happen to him, Greenwald replied that ''media descriptions of it have been overly simplistic. ''It's not just a matter of, if he dies, things get released, it's more nuanced than that,'' he said. ''It's really just a way to protect himself against extremely rogue behavior on the part of the United States, by which I mean violent actions toward him, designed to end his life, and it's just a way to ensure that nobody feels incentivised to do that.''

He declined to provide any more details about the pact or how it would work.

Greenwald said he himself has beefed up his own security, particularly since a laptop went missing from his Rio home.

''I don't really feel comfortable discussing the specific measures, but one would be really irrational and foolish to have thousands of top-secret documents from the most secretive agency of the world's most powerful government and not be thoughtful about added security,''

said the 46-year-old former constitutional and civil rights lawyer who has written three books contending the government has violated personal rights in the name of protecting national security.