US Airways Group Inc.'s flight attendants union asked a federal agency to release it from contract talks with the sixth-largest U.S. airline, saying the two sides can't reach agreement.

The Association of Flight Attendants, which represents about 9,800 US Airways workers, must get permission from the National Mediation Board to end negotiations.

Once the board agrees to end the talks, it offers both sides arbitration. If either side rejects that, a 30-day cooling-off period starts, after which the union can go on strike or the company can impose work rules.

The union said it wants to start the 30-day countdown. Agreement has been reached on 15 of 31 contract sections after about three years of negotiations, the AFA said. Key provisions on pay, pension and benefits haven't been resolved.

"We want a contract, not a strike," said Lynn Lenosky, president of the AFA master executive council for US Airways, in a statement. "But it's unfair that this company has stalled our negotiations for three years."

The Arlington-based airline "will continue to work with our flight attendants toward an equitable contract," US Airways spokesman Richard Weintraub said.

He declined to comment on specifics of the talks.

The National Mediation Board didn't immediately respond to calls for comment.

The flight attendants plan to picket on the issue at 16 U.S. airports on Wednesday, a heavy travel day because of the Thanksgiving holiday.