US authorities on Monday filed an antitrust lawsuit against American Express for allegedly imposing unfair restrictions on merchants that use its cards, the credit card company said.

American Express (AmEx) is accused of unfairly trying to prevent merchants from using other credit cards. The finance company currently mandates that merchants not discourage customers from paying with an AmEx card, even though they charge sellers higher usage fees than their competitors.

"The government's lawsuit claims that terms of American Express merchant contracts, which protect card members against discrimination and disruption at the point of sale, violate US antitrust laws," it said in a statement.

The Justice Department had also reached settlements over similar charges with American Express rivals Visa and MasterCard.

"The terms of the settlement will be announced later today," MasterCard spokesman Jim Issokson told AFP in a statement.

But American Express vowed to fight the charges.

"We have no intention of settling the case," AmEx chief executive Kenneth Chenault said.

"Whatever the intent, the government's new approach would hand an unfair advantage back to Visa and MasterCard," Chenault said.

"The antitrust lawsuit filed today against the company is a significant retreat from previous Department of Justice efforts to promote competition in the payments industry," the statement said.

"The government's one-sided remedy would put more power in the hands of Visa and MasterCard, the networks that steadily increased prices for credit card transactions over the past decade, that control over 70 percent of the market and that have ten times as many cards as American Express."

AmEx expressed confidence that courts would side with them in the matter.

"Because American Express has refused to change its rules, consumers are being held hostage from receiving the expanded choices and lower prices that they deserve under our settlement," Attorney General Eric Holder told reporters on Monday afternoon. "We cannot allow this to stand."

With AFP.