Wal-Mart, the world's biggest retailer, has called on merchants to reject a multi-billion-dollar settlement deal over credit-card fees, saying it was not in their or consumers' interest.

The proposed settlement, announced this month as a resolution of a class-action suit by merchants over so-called "swipe" fees, would deprive merchants of their rights, Wal-Mart Stores said in a statement.

"The proposed settlement would not structurally change the broken market or prohibit credit card networks from continually increasing hidden swipe fees, which already cost consumers tens of billions of dollars each year," it said.

In the proposed settlement announced on July 13, credit card giants Visa and MasterCard agreed to pay more than $US6 billion ($A5.87 billion) to millions of merchants which had sued them for allegedly fixing card-use fees.

Visa agreed to pay $US4.03 billion ($A3.94 billion) while MasterCard, as well as card-issuing banks that were part of the suit, would pay $US2.02 billion ($A1.98 billion), according to New York federal court documents.

Visa and MasterCard would also have to cut their so-called "swipe" fees for eight months that could give the merchants another $US1.2 billion ($A1.17 billion) in relief.

The Arkansas-based Wal-Mart, which operates in 27 countries, charged that the deal "would require merchants to broadly waive their rights to take action against the credit card networks for detrimental conduct or acts".

"We encourage all merchants to put consumers first and reject the settlement."