Credit card giants Visa and MasterCard say a computer hacker has breached a security system, gaining access to at least 5.6 million accounts.
A spokesman for Visa said Tuesday there have been no reports that any of the accounts have been used fraudulently. Officials from MasterCard have not said whether any accounts were used illegally.
Officials say hackers gained access to the accounts by breaking into a secure database belonging to a third-party that processes credit transactions on behalf of merchants.
It is unclear if the hackers were operating in the United States, Canada or overseas.
Both MasterCard and Visa say they immediately notified all card-issuing banks of the breach.
Both card companies have zero-liability policies, which protect card holders from fraudulent or unauthorized charges.
There are more than one billion Visa-marked cards worldwide. MasterCard has some 1.7 billion accounts around the globe.
