WASHINGTON (AFP) - The National Security Agency's website went down for several hours on Friday, but the United States spy service known for hacking into computer networks blamed the outage on a technical mistake.

The website, NSA.gov, went down in the afternoon, setting off speculation on Twitter that the site may have suffered a denial of service attack by hackers.

But an NSA spokesman denied the claims, saying they were "not true".

"NSA.gov was not accessible for several hours tonight because of an internal error that occurred during a scheduled update. The issue will be resolved this evening," the spokesman added.

"Claims that the outage was caused by a distributed denial of service attack are not true."

The website was operational again around 10.30pm local time on Friday.

The hacker group Anonymous joked about the incident in a tweet, without saying if it had played any role. "Aw don't panic about nsa.gov being down. They have a backup copy of the Internet," it said.

The loosely organised, international hacker collective has frequently clashed with the US authorities over file-sharing as well as allowing banks to handle donations to the anti-secrecy group WikiLeaks.

The NSA has been at the centre of a furor over its vast electronic surveillance operations, revealed in a series of leaks from former intelligence contractor Edward Snowden, who has obtained asylum in Russia.