WASHINGTON — The Treasury Department has a message for taxpayers getting calls from the IRS demanding immediate payment: Hang up.

    Those aggressive, threatening calls are coming from phone scammers. And they are successful, too, costing more than 5,000 victims more than $26.5 million since October 2013. About 900,000 scammer contacts were reported during that period.

    The callers fraudulently claim to be IRS officials and demand that taxpayers immediately send them payment. They often have some personal information about the person they're calling and threaten criminal charges unless immediate payment is made.

    "This scam has proven to be the largest of its kind that we have ever seen," said J. Russell George, Treasury Department inspector general for tax administration. "It is critical that all taxpayers continue to be wary of unsolicited telephone calls and e-mails from individuals claiming to be IRS and Treasury employees."

    The agency has responded with public service announcements reminding people that the IRS sends letters rather than calling taxpayers.

    "If someone unexpectedly calls claiming to be from the IRS and uses threatening language if you do not pay immediately, that is a sign that it is not the IRS calling, and your cue to hang up," George said.

    The agency recommends taxpayers who are contacted by phone scammers fill out an "IRS impersonation scam" form on the Treasury inspector general website, www.treasurytigta.gov, or call Treasury officials at (800) 366-4484.