Canadian credit card companies are telling card holders and financial institutions to keep a close eye on all financial transactions following the security breach in the U.S. that could affect up to 240,000 Canadian Visa card holders.

Cardholders are being told to contact their financial institution immediately if they find any unusual activity in their accounts.

MasterCard International Inc. announced Friday that the banks and account numbers of as many as 40 million credit card holders may have been accessed by an unauthorized user.

The security breach was traced to Atlanta-based CardSystems Solutions Inc., which processes credit card and other payments for banks and merchants.

The majority of the nearly 14 million MasterCard card users affected by the scandal are American.

MasterCard and Visa both said affected Canadian customers will not face any financial losses as result of the security breach because both have zero-liability policies when dealing with credit card fraud.