NEW YORK (CNN) - A hacker has gained access to as many as 2.2 million Visa and Mastercard accounts, the two companies announced Monday.

The hacker breached the security system of a company that processes credit card transactions on behalf of merchants, Visa and Mastercard said.

None of the compromised Visa cards has been used fraudulently, said Visa spokesman John Abrams. A Mastercard spokeswoman could not say whether any of their cards had been used fraudulently.

The affected accounts make up less than a third of a percent of the 560 million Visa and Mastercard cards in the United States. Spokesmen for the two companies said they promptly notified the affected banks that issued the cards.

Both card companies have zero liability policies, which protects the card holder from responsibility for any unauthorized or fraudulent charges.

Mastercard and Visa would not disclose how many banks they had notified, nor would they say whether this was a regional or national problem.

Citizens Bank, a financial institution serving the Northeast, shut down the accounts of 8,800 customers whose card numbers had been accessed after being notified by Mastercard last Friday, bank spokeswoman Pamela Crawley said. All of those accounts were safe, she said.

CNN called several leading card issuers, but only one, First USA Bank, returned calls for comment. Thomas Kelly, a spokesman for its parent, BankOne, said FirstUSA's cards were not affected and the bank was not aware of any security problems affecting its customers.

Mastercard said it is working with authorities, including the FBI, to help identify the culprit.