A SCAM involving fake FBI or CIA emails had unleashed a computer virus that has spread rapidly worldwide, US officials and security experts said

The FBI released a statement on its website noting the agency was not the source of the emails. But experts said that the virus was propagating because the authors made the message appear authentic.

The FBI statement said recipients of this or similar messages "should know that the FBI does not engage in the practice of sending unsolicited emails to the public in this manner".

The messages appeared to be sent from an email address such as mailfbi.gov, postfbi.gov, adminfbi.gov or a similar address, and directed the recipient to open an attachment to answer a question. The opening of the file activated the virus and caused it to spread to others.

Internet security firm Sophos said similar emails appeared to come from the Central Intelligence Agency, but both contained a strain of the Sober virus that had been spreading worldwide. In a four-hour period yesterday, the worm "has accounted for over 61 per cent of all viruses reported to Sophos, making it currently the most prevalent virus spreading across the world", Graham Cluley, senior technology consultant at Sophos said.

"This variant of the Sober worm may catch out the unwary as they open their email in box this morning.

"Every law abiding citizen wants to help the police with their enquiries, and some will panic that they might be being falsely accused of visiting illegal websites and want to click on the unsolicited e-mail attachment.

"All users should be reminded to follow safe computing guidelines, and PCs should be kept automatically updated with the latest anti-virus protection."

The email stated: "We have logged your IP-address on more than 30 illegal websites", and directed the recipient to open an attachment to respond to questions.

"The FBI takes this matter seriously and is investigating," the law enforcement agency said, urging those receiving emails of this nature to report it to the Internet Crime Complaint Center via http://www.ic3.gov .

California-based firm PandaLabs said the virus quickly became the most prevalent spreading around the globe.

One reason for its success was that "this new variant uses social engineering techniques, tricking users into running files that contain the system code", PandaLabs said.

The virus used another trick ? displaying a dialogue box saying that no viruses, Trojans or spyware were found, according to PandaLabs, even though the computer was left unprotected against future attacks.

Experts noted each infection caused a computer to send out new copies of the email to those in the computer's address book.

"The propagation capacity of Sober.AH, means that every time there is a new infection, the chances of receiving an infected email increase exponentially," Luis Corrons, director of PandaLabs, said.

PandaLabs and others noted that some of the emails were being delivered in German to addresses in Europe, purportedly coming from the BKA, the German federal police.

The SANS Institute's Internet Storm Centre, an academic industry partnership, urged Internet users to exercise caution because anti-virus programs might not detect the latest versions of malicious programs.

"Antivirus software does not provide any reliable protection against current threats," SANS said. "Viruses like Sober tend to change every few hours well in advance of AV signature updates. The fact that an attachment did not get marked is no indication that it is harmless."