Antivirus company Network Associates Japan Co. said Thursday it has received about 550 reports from Japanese companies whose computers have been infected either with the "Sasser" worm or its variations.

The new worm has been spreading across the world since the beginning of May. Earlier this week, it hit Network Associates clients in Germany, Britain and the U.S.

Computer users need to download antivirus patches for Microsoft operating systems at an early time to repel attacks by variations of "Sasser" that are expected to emerge in the near future, a Network Associates Japan official said.

According to antivirus companies, computer viruses usually appear and spread soon after software companies announce defects in a particular operating system, because viruses designed to attack the defects are quickly created.

"Sasser" can infect computers when they connect to the Internet, and force infected ones to shut down after displaying an error message.

The government on Thursday issued an alert to computer users concerning the worm and its variants, with many Japanese companies expected to resume full operations Monday after the Golden Week holiday period.