Online scams including identity theft schemes, "advance fee" and "romance fraud" cost Americans some $US485 million ($A484.44 million) in 2011, a report prepared for the FBI says.

The Internet Crime Complaint Centre annual report said the number of complaints about online fraud rose 3.4 per cent to 314,246.

The most common complaints included FBI-related scams, in which criminals impersonate the FBI to gain sensitive data, identity theft and advance fee fraud - schemes in which emails pledge to release funds for a transaction fee.

One of the newer schemes involves fraudulent car sales - in which a criminal posts a car for sale at an attractive price, pretending to be desperate to sell before moving or deploying overseas, and then seeking a deposit to hold the vehicle.

Other fraud schemes include the "non delivery" of merchandise bought online or through an auction, and "overpayment fraud" in which someone receives a cheque with instructions to deposit it in a bank account and send excess funds or a percentage of the deposited money back to the sender.

For victims reporting financial losses, the average was $US4187 ($A4182), said the crime centre, a partnership of industry and the FBI.

The centre in 2011 received over 5,600 complaints of "romance scams" in which scammers target individuals searching for companionship or romance online.

"Victims believe they are 'dating' someone decent and honest. However, the online contact is often a criminal with a well-rehearsed script that scammers use repeatedly and successfully," the report said.

"Scammers search chat rooms, dating sites, and social networking sites looking for victims. Although the principal group of victims is over 40 years old, divorced or widowed, disabled and often elderly, all demographics are at risk."

These scams often result in both monetary and emotional distress, the report said. On average, each victim reported a loss of $US8900 ($A8,889.78), for a total of $US50 million ($A49.94 million).

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