American Express Co. said it's appealing a federal judge's order approving Visa Inc. and MasterCard Inc.'s $5.7 billion settlement that ended years of litigation with U.S. merchants over allegations that credit-card swipe fees are improperly fixed.

American Express, in a filing Jan. 8 in federal court in Brooklyn, N.Y., joined retailers and trade associations including Wal-Mart Stores Inc., Amazon.com Inc., 7-Eleven Inc. and Barnes & Noble Inc. The settlement potentially applies to claims of competitors and not just merchants, American Express said.

"In our primary business we are not a merchant, but a card network, card issuer and merchant acquirer in which we are a competitor to Visa, MasterCard and their issuing and acquiring banks," Marina Norville, a spokeswoman for Amex, said in an e-mailed statement.

Amex does act as a merchant that accepts Visa and MasterCard cards as part of its travel and prepaid products business, she said.

U.S. District Judge John Gleeson approved the accord on Dec. 13, saying he was satisfied with the settlement, which was estimated to be the largest-ever U.S. antitrust accord.

The case is In re Payment Card Interchange Fee and Merchant Discount Antitrust Litigation, 05-md-01720, U.S. District Court, Eastern District of New York (Brooklyn).

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