The FBI is warning internet users to beware of a virus masquerading as an email from the US law enforcement agency.

The scam emails tell the recipients that their surfing has been monitored by the FBI's Internet Fraud Complaint Centre and that they have accessed illegal websites.

Recipients are urged to open an attachment and answer some questions, but the attachment contains a computer virus.

"These emails did not come from the FBI. Recipients of this or similar solicitations should know that the FBI does not engage in the practice of sending unsolicited emails to the public in this manner," the agency said.

Opening email attachments from an unknown sender is a "risky and dangerous endeavour", it warned, as such attachments frequently contain viruses. "The FBI strongly encourages computer users not to open such attachments." The agency said that it is taking the matter seriously and is investigating. Users receiving emails of this nature are encouraged to report it to the Internet Crime Complaint Centre here.