But an NSA spokesperson denied the claims, saying they were "not true. NSA.gov was not accessible for several hours because of an internal error that occurred during a scheduled update. The issue will be resolved this evening.”
He added, "Claims that the outage was caused by a distributed denial of service attack are not true."
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 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.