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 Visa accounts has been used fraudulently, Visa spokesman John Abrams said.

The affected accounts make up about one-third of 1 percent of the 560 million MasterCard and Visa cards in the United States. Spokesmen for the two companies said they have notified the banks that issued the affected cards.

Both card companies have zero-liability policies, which protect cardholders from being held responsible for unauthorized or fraudulent charges.

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