A mass email claiming to be from the US FBI or CIA is circulating online, carrying an attachment containing a variant of the Sober computer virus.

In an announcement Tuesday, the FBI advised computer users that the agency never sends unsolicited emails and that they should not open the attachments in the fraudulent messages.

The scam email tells recipients that their Internet use has been monitored by the FBI (or CIA, Macworld has learned) and that they have accessed illegal websites, according to the FBI. The emails are sent from addresses including [email protected], [email protected] and [email protected] They direct recipients to open an attachment and answer questions.

The text reads: "We have logged your IP-address on more than 30 illegal websites. Important: Please answer our questions! The list of questions are attached." The messages then include a fake name and the real address and phone number of the agency.

Catherine Milhoan, an FBI spokeswoman, said that recipients of the hoax emails began calling the FBI's Internet Crime Complaint Center yesterday to ask if they were legitimate. About 4,000 calls were received by last night, with some callers saying they had already clicked on the attachment. Others were cautious and wanted to check with the FBI before opening the attachment.

An investigation into the incident is continuing. Recipients of the fake messages are being asked to report them online to the Internet Crime Complaint Centre.