People who have recently travelled to the United States are being warned to check their credit card statements carefully after cyber criminals reportedly stole the data of more than 10 million Mastercard and Visa users.

It is believed the cyber criminals stole the information of millions of customers over several months from a US payment processor.

The credit card companies say they are investigating the breach, and will be issuing new cards to anyone who has been affected.

Discover Financial Services was also the hit in the latest in a string of incidents that have put the personal information of millions of credit cardholders at risk.

The companies, which represent three of the four largest global credit card processors, said the issue stemmed from a third-party vendor and not their own internal systems.

It is unclear how large the breach is and how many customers might be affected.

MasterCard and Visa said they have notified banks about the security breach. Discover said it is monitoring accounts for suspicious activity and will reissue cards "as appropriate."

MasterCard said it notified law enforcement officials and has hired an independent data-security organisation to review the possible breach.

A US Secret Service spokesman said it was investigating but declined to give any specifics about the breach.

Visa said it provided banks with affected customers' account numbers and emphasised that customers are not responsible for fraudulent purchases.

ABC/wires