A computer hacker, or hackers, has gained access to the credit card numbers of as many as five million credit card customers.
As a result, information was stolen from more than 2.2 million MasterCard International accounts and approximately 3.4 million Visa USA cardholder accounts, according to those companies.
American Express cardholder account information was also accessed, according to Christine Elliott, a corporate spokeswoman for American Express.
Elliott declined to comment on how many American Express accounts were affected.
The theft occurred when the system of a company that processes credit card transactions for merchants was broken into, according to statement released by Visa USA.
Systems operated by Visa and MasterCard were not compromised, and both companies said they have contacted all the financial institutions affected by the theft.
No information was available about which banks were affected by the theft, but a Visa spokesman said that none of the stolen Visa account information has been used fraudulently.
MasterCard is continuing to investigate whether the account information has been used to make purchases, according to Sharon Gamsin, vice president of global communications at MasterCard.
Neither Visa nor MasterCard would identify the company that was hacked, nor would they provide information on how the theft occurred, citing security concerns.
MasterCard became aware of the security breach during the week of February 3, Gamsin said.
Visa and MasterCard, like other credit card companies, use third party companies to manage credit card transactions between merchants and the financial institutions that issue the credit cards.
Those companies transfer money to a merchant's account and manage any transaction fees, according to a Visa spokesman.
Because a single vendor handles transactions from a number of different companies, however, it is likely that the thefts were not limited to Visa and MasterCard accounts, according to Gamsin.
The US Federal Bureau of Investigation's (FBI's) Cybercrime division is also investigating the theft, according to Cybercrime division spokesman Bill Murray.
The decision about whether to cancel a credit card account or merely flag it for possible fraud is up to the financial institutions that issued the card, according to Gamsin.
Citizens Bank NA was notified of the crime by MasterCard on Friday and immediately cancelled the affected credit cards to protect its customers, according to Barbara Cottam, director of corporate communications at Citizens Bank, which is part of Citizens Financial Group of Providence, Rhode Island.
Although Cottam would not say how many Citizens Bank customers were affected, she said that published reports putting the number at 8800 were accurate.
Citizens Bank called those customers and issued new credit cards to them, according to Cottam.
"The cards are in the mail," Cottam said.
Cottam said she did not know of any fraudulent transactions affecting Citizens Bank customers that were linked to the theft.
In contrast, Elliott said that American Express would not cancel credit cards attached to compromised accounts, but that it had implemented a process to detect fraudulent activity on the affected cards.
Elliott would not provide details on that process, but said the company will notify specific card members if fraudulent activity is detected and action needs to be taken.
As of Tuesday, American Express was not aware of any fraudulent activity connected to the compromised accounts, Elliott said.
Both Visa and MasterCard offer zero-liability policies, which absolve cardholders of responsibility for unauthorised purchases.
Cardholders who discover that their account has been used fraudulently should contact the financial institution that issued the card, Gamsin said.