WASHINGTON (AP) — US Airways and American Airlines say they will use "all legal options" to fight back against a challenge to their merger plans.

The Justice Department Tuesday joined a half dozen states in filing a lawsuit to block the merger in federal court in Washington. The government says the merger will cause "substantial harm" to consumers by leading to higher fares and fees.

The two airlines say the government's conclusions are wrong. In a statement, they say they would create a stronger network of flights that would give travelers more choices.

Analysts say the Justice Department could be looking for more time and leverage to squeeze out some concessions. Many experts had expected regulators to push American and US Airways into giving up some takeoff and landing slots at Reagan National Airport outside Washington, allowing for new competitors there.

Consumer advocates are welcoming the lawsuit. Charlie Leocha, who heads the Consumer Travel Alliance, says it's "the best news that consumers could possibly have gotten."