CONSUMER ALERT — Mastercard and Visa are warning banks about a possible security breach at a U.S. based processor.

Officials say the breach could affect millions of Visa and Mastercard credit card holders.

Both companies are investigating the breach.

Mastercard officials issued a statement saying law enforcement has been notified of the matter, and an "independent data security organization" is conducting an ongoing forensic review.

The number of cardholders, banks or issuing companies affected has not been released.

Both companies are investigating the breach.