NEW YORK (CBSMiami/AP) – MasterCard and Visa are investigating a potential security breach that may affect ten million cardholders.

The companies, which are the two largest global credit card processors, has alerted bank and law enforcement officials and said the breach is related to a third party vendor. The company is advising cardholders who have concerns to contact the bank that issued their cards.

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Credit card companies generally protect customers against fraudulent transactions, and Visa said specifically Friday that its U.S. customers were not at risk.

Both Visa and MasterCard said their own systems had not been compromised.

“The incident is currently the subject of an ongoing forensic review by an independent data-security organization,” a MasterCard spokesman said in a statement. “MasterCard is concerned whenever there is any possibility that cardholders could be inconvenienced, and we continue to both monitor this event and take steps to safeguard account information. “If cardholders have any concerns about their individual accounts, they should contact their issuing financial institution.”

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Discover Financial Services said it is also monitoring accounts for suspicious activity and will reissue cards “as appropriate.”

The breach reportedly took place between Jan. 21, 2012, and Feb. 25, 2012, and involved data that could be used to counterfeit cards, according to the blog Krebs on Security.

MasterCard said it notified law enforcement officials and has hired an independent data-security organization to review the possible breach. A U.S. Secret Service spokesman said the agency was investigating, but declined to give any specifics about the breach.

Last June, hackers stole information for 360,000 credit card accounts at Citigroup.

In the past year, there have been high-profile data attacks against the International Monetary Fund, National Public Radio, Google and Sony’s PlayStation Network.

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