NINE of the 13 computer servers that manage global internet traffic were crippled by a powerful electronic attack on Monday, officials said.

But most internet users didn't notice because the attack only lasted an hour. Its origin was not known, and the FBI and White House are investigating.

One official described the attack as the most sophisticated and large-scale assaults against these crucial computers in the history of the internet.

Service was restored after experts enacted defensive measures and the attack suddenly stopped.

Although the Internet theoretically can operate with only a single root server, its performance would slow if more than four servers failed for any appreciable length of time.