The FBI has warned people not to open unsolicited emails that look like they come from the law enforcement agency, saying they are instead a scheme to spread a computer virus.

In a statement released late on Tuesday, the US Federal Bureau of Investigation said the emails suggest that the recipients accessed illegal websites but actually include attachments that are viruses.

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

The bureau went on to say that opening email attachments from unknown senders "is a risky and dangerous endeavour, as such attachments frequently contain viruses that can infect the recipient's computer".

The FBI is investigating the scam.