More than nine million computers around the world have been infected by a digital virus in what experts believe is a multi-staged attack.

The world leading security experts paint a scary scenario, stressing that they are yet to identify who programmed it and what the next stage will be.

Known as Conficker or Downadup, it is spread by a recently discovered Microsoft Windows vulnerability, by guessing network passwords and by hand-carried consumer gadgets like USB keys, the New York Times said on Friday.

Experts were quoted as saying that it is the worst infection since the Slammer worm exploded through the Internet in January 2003, and it may have infected as many as nine million personal and business computers around the world.

Worms like Conficker not only ricochet around the Internet at lightning speed, they harness infected computers into unified systems called botnets, which can then accept programming instructions from their clandestine masters.

"If you're looking for a digital Pearl Harbour, we now have the Japanese ships steaming toward us on the horizon," Rick Wesson, chief executive of Support Intelligence, a computer security consulting firm based in San Francisco, was quoted as saying by the paper.

The experts were quoted as saying that many computer users may not notice that their machines have been infected.

But they might be waiting for the instructions to materialise, to determine what impact the botnet will have on PC users.