A fast-spreading computer worm has corrupted corporate computer networks and personal computers in an outbreak that could be more widespread and damaging than the Code Red infections, computer security experts said.

Known as "Nimda," the word "admin" spelled backwards, the worm first appeared in the United States on Tuesday, spread to Asia overnight and thousands of European businesses opened business Wednesday morning with infected computer systems.

Internet security experts had warned of the potential for an increase in virus activity after last week's attacks on the World Trade Center and Pentagon, but U.S. Attorney General John Ashcroft said there was no sign of a link to those events.

"There is no evidence at this time which links this infection to the terrorist attacks of last week," Ashcroft said.

Ashcroft said Nimda could prove "heavier" than the Code Red worm that caused an estimated $2.6 billion in clean-up costs after outbreaks in July and August.

One victim was German electronics conglomerate Siemens AG. The worm infiltrated part of its computer network, a company spokesman said, forcing the firm to shut down some computer servers and its e-mail system for a few hours on Wednesday.

As of 4:30 a.m. EDT, the firm had fortified the affected systems. "No concrete damage was found," the spokesman said, adding the disruption had no impact on business operations.

The Nimda worm spreads by sending infected e-mails that carry an attachment labeled "readme.exe." It also propagates by infiltrating unsecured Web sites and attaching itself to an unsuspecting computer user's Web browser, IT officials said.

Its target is personal computers and Microsoft computer servers, making it a more malicious and versatile strain than earlier Internet threats, experts said.

In Europe, more than 15,000 companies had been infected by Nimda, said Raimond Genes, vice president of sales and marketing for Trend Micro, a security software firm. "This one is really horrible," he said. "It's a combined attack."

The affected companies, which he would not name, are located in Germany, the United Kingdom, France, Italy and Switzerland, said Genes.

Graham Cluley, senior technical consultant for Sophos Anti-Virus in Oxford, told Reuters on Wednesday he would not be surprised if hundreds of thousands of users had been affected.