A major security breach at an unnamed major retailer may have exposed hundreds of thousands of consumers in the northeastern U.S. to fraud.

Close to 70 Massachusetts banks have been contacted by Visa and MasterCard about the incident, which occurred between December and March, the Massachusetts Bankers Association (MBA) said Monday in a statement.

"The MBA estimates that hundreds of thousands of credit and debit cards owned by consumers in Massachusetts and northern New England states could be affected, and is urging consumers to monitor their accounts," the bankers association said.

The group did not provide any details on how the breach occurred or name the "major retailer" responsible for the breach.

However, MasterCard characterized the incident as a "potential security breach," and issued a statement saying that the matter is being investigated by law enforcement. Because of the ongoing investigation, however, the credit-card company declined to provide additional details.

The largest retailer breach to date in the U.S. occurred over a year and a half period starting in 2005, when hackers gained access to computer systems at Massachusetts-based TJX Companies, owners of T.J.Maxx, Marshalls and Bob's Stores. That breach affected more than 94 million credit- and debit-card accounts.