"We're getting inundated with complaints," said Donna Gregory from the US Internet Crime Complaint Centre, referring to the virus known as Reveton ransomware, which has hit users in the US and globally.
"Some people have actually paid the so-called fine," she said, describing the virus as "drive-by malware" that installs itself when a user clicks on a compromised website and issues a message saying a federal law has been broken.
"The bogus message goes on to say that the user's internet address was identified by the FBI or the Department of Justice's Computer Crime and Intellectual Property Section as having been associated with child pornography sites or other illegal online activity," the FBI said in a statement.
"To unlock their machines, users are required to pay a fine using a prepaid money card service," it said, accompanied by reports from victims who had paid fines of $200.
The Internet Crime Complaint Centre is a partnership between the FBI and the National White Collar Crime Centre and it was launched in 2000 to allow people to report cyber crimes to US law enforcement.
Gregory, however, said there is no easy fix for users whose computers have been infected.
"Unlike other viruses, Reveton freezes your computer and stops it in its tracks. And the average user will not be able to easily remove the malware," she said.