A new computer virus called "bot," which creates networks of infected computers to mount attacks on specific Web sites, is spreading, National Police Agency officials said Saturday.
The NPA has been monitoring the virus around the clock, calling on computer users to be aware of possible infection.
Bot, named after a robot, infects computers and eventually forms a network of infected computers without being noticed by their users.
When ordered by hackers, bot uses the network to send out spam or invalid data, disabling other computers and targeted Web sites.
The NPA has already confirmed 20 infected networks. One case involves about 30,000 infected computers.
There have been no reports of damage at Japan-based Web sites.
In Britain, a commercial gambling Web site was threatened by hackers who said they would destroy the site unless its operator paid them off, the officials said.
The NPA advises computer users to use basic antivirus software and corrective service programs for Windows and other operating systems.