Hackers may have seen credit card details belonging to as many as five million MasterCard and Visa users in the US, according to reports.
News agency Reuters said that the companies had admitted that around 3.4 million Visa and more than two million MasterCard accounts in the US had been accessed via the systems of an unnamed third party which processes credit card transactions.
Both companies claimed that they were yet to see any fraudulent transactions on the exposed cards.
The affected banks were notified last Friday, but Visa and MasterCard declined to give details of which banks had issued the exposed cards.
Both credit card firms have zero-liability policies, which means that customers will not be held responsible for unauthorised or fraudulent charges.
The FBI has been contacted over the hack.
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