In the most recent form of online defacement, hackers are impersonating the FBI and claiming the authority to shut down websites.
Late last week a "handful" of sites were hacked and then defaced with a US FBI (Federal Bureau of Investigation) seal as well as an antipiracy warning claiming the site had been seized by the law enforcement organisation, said Paul Bresson, an FBI spokesman.
The sites being hacked were mostly gaming sites, he said.
Bresson declined to say what websites had been hit with the attacks, and he couldn't say what might have prompted these hacks. "I'm not sure if it's financially motivated," he said. "I'm not sure what their motive would be."
In some cases, hackers also changed domain name registration information to make it look as if the FBI now owned the website, he added.
Although these attacks are not widespread, the FBI is concerned any time its name is misused, Bresson said.
A sparsely worded advisory on the attacks was published on Friday by the Internet Crime Complaint Center, a joint effort between the FBI and the National White Collar Crime Center.