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.

The National Security Agency, which Mr 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.

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

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

Deputy Defence Secretary Ashton Carter said: "We normally compartmentalise information for a very good reason, so one person can't compromise a lot."

"Loading everything onto one server ... it's something we can't do. Because it creates too much information in one place."

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

Mr 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 disclosures by Mr Snowden, who is wanted on espionage charges, have raised Americans' concerns about domestic spying and strained relations with some US allies.

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

Mr 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 Mr Snowden had roughly 200 classified documents.

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

Current and former US officials said on condition of anonymity that while authorities now thought they knew what documents Mr Snowden accessed, they were not yet entirely sure of all that he downloaded.