"Visa will continue working with its member financial institutions, merchants and appropriate authorities to do whatever is necessary to protect cardholders," Visa said.

Kayce Bell, chief operating officer at Alabama Credit Union (ACU) in Tuscaloosa, Ala., said the company is reissuing cards to about 500 credit card and debit card holders as a result of the breach. The credit union was alerted to the problem last week by Credit Union National Association Inc., she said.

"We received information through our national reporting service that there had been a very large breach of data at Sam's Club," Bell said. About 500 debit cards and credit cards issued by ACU were among the accounts compromised in this incident, she said.

This isn't the first time this year the credit union has had to block and reissue credit and debit cards at Visa's request. Earlier this year, the ACU had to deactivate and reissue about 1,550 cards after Visa notified it that cards compromised in a CardSystems Inc. breach in June were being used fraudulently.

'I find the breach at Sam's Club to be quite surprising,' said Corinne Sherman, vice president of card services at the Pennsylvania Credit Union Association. What is especially of concern is that Sam's Club appears to have stored information from both tracks of the magnetic stripe on the back of credit cards, thereby raising the potential for data thieves to create counterfeit cards, she said.