NEW YORK (CNNMoney.com) - The Internal Revenue Service is warning taxpayers about a so-called "phishing" scam in which criminals are attempting to steal money by sending fraudulent e-mails purporting to be from the IRS.

The agency says that the Treasury Inspector General for tax administration has reported that it found 12 separate Web sites in 11 different countries hosting variations on this scheme in which a site that appears to be run by the IRS asks taxpayers to turn over confidential financial information in order to claim a refund.

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The agency said the fraud begins with e-mails claiming to come from tax-refunds@irs.gov, admin@irs.gov or other variations on the irs.gov theme. The agency said that hundreds of thousands of fraudulent e-mails have been sent to potential victims advising the recipients that they are under investigation by the IRS or that they have a refund pending from the IRS.

The e-mails generally contain a link to a fraudulent Web site, which the recipients are told to visit.

"The site, a clone of the IRS Web site, displayed an interactive page similar to a genuine IRS one; however, it had been modified to ask for personal and financial information that the genuine IRS interactive page does not require," the agency statement said.

The agency warns that as "a general rule, the IRS does not send out unsolicited e-mails or ask for detailed personal information. Additionally, the IRS does not ask people for the PIN numbers, passwords or similar secret access information for their credit card, bank or other financial accounts."

The agency said that another scheme being used by criminals sends a bogus IRS letter and Form W-8BEN to non-residents. That is a real form that is used by financial institutions to establish appropriate tax withholding for foreign individuals. But the fraudulent version of the form asks non-residents to provide personal information such as account numbers, PINs, mother's maiden name and passport number, none of which is asked for on the legitimate form.

The agency said the scams are effective because so many people have previously provided the IRS some financial data.

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