As many as 240,000 Canadians who have a Visa credit card could be affected by a massive security breach in the United States.

It's not clear exactly which Canadian cards might have been affected by the problem at a U.S. company, CardSystems Solutions, which processes credit card payments.

On Friday, U.S. card companies said an unauthorized person may have accessed the accounts of up to 40 million credit-card holders.

A spokeswoman for Visa Canada said it is watching the accounts which may have been affected, but she also said cardholders should monitor their accounts for unusual activity.

Canadian cardholders are protected from fraud through VISA's zero liability program, the company said in a statement.

"We regret any anxiety that this may cause our Canadian cardholders."

The U.S. breach may have been the worst ever in terms of numbers.

• FROM JUNE 17, 2005: Hacker accesses files at Equifax

In Canada this week, a computer hacker accessed the files of about 600 consumers at Equifax Canada, a major credit bureau.