The FBI warned U.S. Web sites and Internet service providers to be on guard for possible denial-of-service attacks Tuesday, but any damage appeared to be minimal as of midday.

The warning, issued Monday night by the bureau's National Infrastructure Protection Center, said that the center had received "credible, but nonspecific information that wide-scale hacker attacks against U.S. Web sites and Internet service providers (ISP) are being planned for later tonight, possibly emanating from Western Europe."

Web site and ISP administrators should "heighten their awareness of network traffic during this period" and "report suspected malicious activities to their local FBI office or the NIPC and to other appropriate authorities," the bulletin said.

Denial-of-service (DoS) attacks are intended to disable Web sites and ISPs by barraging addresses with traffic. A wave of attacks crippled major Web sites such as Yahoo two years ago. More recently, the Code Red worm generated a string of attacks earlier this year.

Key companies measuring Internet performance said attacks were limited and had minimal impact on Web functioning. Matrix NetSystems saw an increase in traffic from Middle Eastern countries starting around 1 a.m. PDT Tuesday and tapering off a few hours later, according to Abelardo Gonzalez, senior Internet analyst for the security and Internet monitoring company.

The number of reachable sites monitored by the company dropped about 1 percent during the period, he said.

"The overall effect was pretty minimal," he said. "Everything was back to normal" by 6 a.m. PDT.

A representative of Keynote Systems, which measures Internet performance, said that company had observed no unusual activity since Monday evening.