A group of Serbian hackers are suspected of being behind attacks on more than 50 Web sites.
The sites, which include Adidas, Pfizer, Metro Goldwyn Mayer and Manchester United, had their contents removed and instead displayed a crest and a banner reading "Kosovo is Serbia".
Other sites attacked include slovenia.com, croatia.com and bosnia.com.
According to Web monitoring company WebDNS, the attack started on Sunday night. Alex Jeffreys, technical director of WebDNS, said he first noticed the attack when a large number of company domains appeared to have changed ownership.
He said it appeared as if the hackers had impersonated e-mails requesting a change of address using an automated service.
All the domains were registered with a single company - Network Solutions. A spokeswoman for Network Solutions said a small number of domain names were subject to attack.
Domain names that had been attacked or "hijacked" were those whose registrants had subscribed to MAIL-FROM, the lowest protection scheme available for a domain name record.
"We immediately took steps to prevent further unauthorised changes," she said. "To the best of my knowledge, we have detected all the unauthorised modifications and corrected the discrepancies."