NEW YORK (AP) - American Express increased interest rates on some of its credit card accounts by an average of 2.5 percentage points in recent weeks, a spokeswoman for the company said Wednesday.

Spokeswoman Elizabeth Crosta declined to say how many of its roughly 42 million cards would be affected. But she said Wednesday that it was a "small percentage" of its customers.

American Express had been charging a lower interest rate, as much as 3.25 percentage points, on its credit cards compared with rates its competitors charge for customers with similar credit scores, Crosta said. The rate increase was authorized after an analysis showed the difference.

Customers affected will be notified and will have the option to keep the card at the higher interest rate or cancel their cards. The new interest rate only affects new purchases and balance transfers.

American Express' move is unusual for the industry. Credit card rates have been going down, not up.

Credit card companies decide what interest rate to charge their customers based a number of factors. Two big factors are the bank's cost of borrowing and the chance a customer may not be able to pay his or her credit card.

The Federal Reserve has kept its benchmark interest rate at near zero since 2008. That is partly what credit card companies use to calculate what rate to charge their customers. However, it is possible the bank might raise interest rates from near-zero levels as soon as this summer.

Also credit card delinquencies have remained stable or have fallen. A report from credit agency TransUnion showed U.S. credit card delinquency rates were 1.47 percent in the fourth quarter of 2014, effectively unchanged from 1.48 percent in 2013.

"When (the Fed raises rates), most Americans will see their credit card interest rates increase almost immediately," said Matt Schulz, an industry analyst for, in an email. "Other than that, however, it's unlikely that rates will increase much, if at all, in the near future. The marketplace is just too competitive right now to allow that to happen."

However, American Express has had a tough couple of months, and that could explain the reason for its increase in some card rates.

Last week, a federal judge ruled that American Express violated U.S. anti-trust laws barring merchants from asking customers to use one credit card over another. And in late January, the company disclosed that it was ending its exclusivity agreement with Costco Wholesale Corp., which had been one of American Express' biggest merchants. JetBlue Airways Corp. is also ending its arrangement with AmEx.

The majority of American Express' cards are charge cards, where there is no interest because the balance has to be paid off each month. With its Costco deal ending, the company is likely searching for new sources of revenue. Higher interest rates are one way credit card companies can increase revenue, but it can come at a cost because customers could use their credit cards with higher interest rates less.

"While closing the account might seem like a headache, the credit card marketplace is so super-competitive these days that you should be able to find a new card without too much trouble," Schulz said.