US steps up security

The disclosures by Snowden raised America's concerns about domestic spying and strained US relations.

US National Security Agency whistleblower Edward Snowden, an analyst with a US defence contractor during an interview with the Guardian in Hong Kong. Picture :AFP

ASPEN, COLORADO - The United States is overhauling procedures to tighten access to top-secret intelligence in a bid to prevent another mega-leak like the one carried out by former spy agency contractor Edward Snowden, senior US officials said on Thursday.

The National Security Agency, which Snowden worked for as a Hawaii-based contractor, said it would lead the effort to isolate intelligence and implement a "two-man rule" for downloading - similar to procedures used to safeguard nuclear weapons.

"When are we taking countermeasures? ... The answer is now," Deputy Defense Secretary Ashton Carter told the Aspen Security Forum in Colorado.

NSA Director General Keith Alexander told the forum the two-man rule would apply to system administrators like Snowden and anyone with access to sensitive computer server rooms.

"You limit the numbers of people who can write to removable media," Alexander said. "Instead of allowing all systems administrators (to do it), you drop it down to a few and use a two-person rule."

"We'll close and lock server rooms so that it takes two people to get in there."

Carter partly blamed the security breach on the emphasis placed on intelligence-sharing after the wake of the September 11, 2001, attacks, which eventually allowed someone like Snowden to access so many documents at once.

"We normally compartmentalize information for a very good reason, so one person can't compromise a lot," Carter said. "Loading everything onto one server ... it's something we can't do."

Alexander said Snowden had been trusted with moving inside networks to make sure the right information was on the computer servers of the NSA in Hawaii.

TIP OF THE ICEBERG?

Snowden fled to Hong Kong in May, a few weeks before publication in Britain's Guardian newspaper and the Washington Post of details he provided about secret US government surveillance of Internet and phone traffic.

The 30-year-old had his US passport revoked, and is still stuck in the transit area of Moscow's Sheremetyevo airport and has applied for temporary asylum in Russia.

Alexander declined to say how many documents Snowden took, but when asked whether it was a lot, he said, "Yes."

Carter said the assessment was still being conducted, but "I can just tell you right now the damage was very substantial."

Senator Dianne Feinstein, who chairs the Senate Intelligence Committee, said last month that US officials advised her that Snowden had roughly 200 classified documents.

But American officials and others familiar with Snowden's activities say they believe that at a minimum, he acquired tens of thousands of documents.

Officials said Snowden was skilful at going into areas and then covering his tracks, which posed a challenge in trying to determine exactly what materials he had accessed.

Alexander also said it would take time to implement across the Pentagon and the broader US intelligence community.

Among US allies, German Chancellor Angela Merkel is under pressure to toughen her stance on the US program.

Alexander said the program had helped European allies including Germany, France and Denmark, without offering details. Asked about his reaction to German expressions of surprise, Alexander stated: "We don't tell them everything we do or how we do it. Now they know."