WASHINGTON (AFP) - Valentine's Day e-greetings from a stranger could deliver more than the recipient bargained for in the shape of a destructive "Trojan horse" that hijacks computers, the FBI warned Tuesday.

"If you unexpectedly receive a Valentine's Day e-card, be careful," the Federal Bureau of Investigation said in a statement, warning Internet users to "be on the lookout for spam emails spreading the Storm Worm malicious software (malware)."

"The Storm Worm virus has capitalized on various holidays in the last year by sending millions of emails advertising an e-card link within the text of the spam email. Valentine's Day has been identified as the next target," the FBI said.

The bogus email directs the recipient to click on a link to retrieve an electronic Valentine's Day card.

If the user falls for the ruse, malware will infect their computer or the device they used to connect to the Internet, and make it become part of a Storm Worm botnet, according to the FBI.

A botnet is a network of computers that, unbeknownst to their users, forward transmissions, such as spam, viruses and malware, to other computers linked to the Internet.

Botnets can also be used in identity theft, the FBI warned, urging Internet users not to open Valentine's greetings from unknown senders.