Update: Several search engines, including Google, were affected on Monday as the worm attempted to use them to search for more targets

Major Internet search engines were crippled on Monday by a variant of the MyDoom worm, which rendered Google inaccessible to many users and slowed results from Yahoo.

The attack also affected smaller engines, including Yahoo's AltaVista and Lycos.

Google representatives confirmed that the MyDoom worm affected performance of the search engine. Despite numerous email complaints received by ZDNet UK sister site CNET News.com that Google's search engine was down for hours, the company down played the impact of the worm and said it had been not widely affected by the worm.

"At no point was the Google Web site significantly impaired, and service for all users and networks is expected to be restored shortly," the company said in a statement.

A Lycos representative said the company is aware of the problem and is working to block the performance obstacles. A Yahoo spokeswoman said the effect on Yahoo was limited due to the company's backup procedures but acknowledged that AltaVista, a search engine that Yahoo owns but relies on different technology, experienced more problems.

"It is kind of an inadvertent DDoS (distributed denial-of-service) attack," said Craig Schmugar, a virus researcher at McAfee Avert. The sites are being knocked out in the search for more email addresses. This is a twist on MyDoom: earlier variants looked for email addresses on the host hard drive. The latest version is now running queries on search engines.

"Once infected, a machine might send thousands of requests," Schmugar said. If a computer is only a few years old and connected to the Internet via broadband, the user probably won't notice the slight decrease of his machine's performance, he speculated.

Internet performance management company Keynote Systems said it noticed on Monday morning a dip in the average performance of the top 40 most popular Web sites it measured. The availability of those sites rarely dips below 97 percent, according to Della Lowe, a Keynote representative. However, since 7 a.m. (PST), reliability dipped 1.5 percent, and the company said it believes that the MyDoom variant was causing the overall slowdown.

"That suggests there could be some wider Internet problem, but we're still researching the data," Lowe said.

Google users were especially taken aback by the search outage. Todd Breslow of Philadelphia was hoping to use Google to search for the proper way to manipulate image border colours when doing Web development but instead had to turn to MSN's search to complete his query.

"I have just come back from lunch, and Google is STILL not reachable," Breslow said in an email interview. "General thoughts in the office are shock and awe."

The problems were occurring just as Google set the price range -- $108 to $135 (£59 to £73) per share -- for its initial public offering. Google filed on Monday with the US Securities and Exchange Commission to raise as much as $3.3bn. The company is issuing 24.6 million shares.

The company plans to list its shares under the ticker GOOG, according to the filing, and has already registered to trade on the Nasdaq exchange.

No date has been set for the IPO.

CNET News.com's Ina Fried contributed to this story.

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