Wal-Mart Stores Inc., the world's largest retailer, asked a U.S. appeals court to throw out a ruling in a discrimination lawsuit that allows 1.6 million female employees to sue the company as a group.

The class-action lawsuit claims that, since 1998, female employees at Wal-Mart have been paid less than men and offered fewer promotions. Last month a federal judge ruled that the women could pursue their case as a group, which is more efficient for the plaintiffs and provides leverage for a possible settlement.

The lawsuit is the largest civil rights class action ever certified against a private employer, lawyers for the employees said. They estimate that Wal-Mart could be liable for an award of more than $1 billion if the workers win the case. Wal-Mart denies that it discriminated against female employees.

"This court should review this unprecedented, unmanageable and unconstitutional class now, before the parties and district court are forced to devote vast amounts of time and resources litigating an action that would not in the end survive judicial review," Wal-Mart said in a petition filed with the U.S. Court of Appeals for the 9th Circuit in San Francisco.

In the petition, Wal-Mart said that pay and promotion decisions are made by individual supervisors and that the women who first filed the suit had failed to show that their experiences were typical or common for all the potential class members.

"Wal-Mart's employment decisions, as carried out by thousands of local managers, indisputably did not affect each and every one" of the class members, the filing said.

Joe Sellers, an attorney for the workers, said that the Bentonville, Ark., company's complaints don't warrant overturning the ruling by U.S. District Judge Martin J. Jenkins in San Francisco.

"In a pattern and practice case, it's not necessary to prove each member of the class was subject to discrimination," Sellers said.

The plaintiffs get seven days to file an answer to the request. A three-judge panel will decide whether to review the class-certification ruling, said Cathy Catterson, the appeals court's clerk.

Six current and former employees claim in the suit that Wal-Mart doesn't pay female workers at the same level as men or give them the same advancement opportunities that male employees receive.

Shares of Wal-Mart rose 15 cents to close at $52.08 in New York Stock Exchange composite trading. The share price has fallen 4 percent since the class-action ruling was disclosed.