Members of American Airlines’ mechanics union rejected a contract proposal and authorized union leaders to call a strike, the Transport Workers Union (TWU) said.

The union said on its website late on Tuesday that two American Airlines work groups represented by the TWU — the mechanics and the stock clerks — voted down a provisional pay deal.

A third bargaining unit — the technical specialists — approved their deal.

“Our members have spoken loud and clear,” said TWU International Vice President Garry Drummond, who heads the union’s Air Transport Division.

“After four years of negotiations, a majority of TWU members … were not convinced that this agreement represented an adequate return for the hundreds of millions of dollars of sacrifices we agreed to in 2003, to keep American planes in the air and prevent our employer from filing for bankruptcy,” Drummond said.

American’s parent, AMR, restructured itself in 2003 with help from labor groups that made concessions to keep the airline in business. The company reorganized out of bankruptcy, while several of its peers like United Airlines parent UAL Corp did so under court protection and won deeper cuts from their workers.

As a result, AMR complains that labor costs by some measurements are higher than those of its rivals. AMR is also in labor talks with other work groups including pilots and flight attendants.

The TWU said on its website that 6,074 mechanics and related members of the union rejected the tentative agreement while 3,371 members voted in favor.

Members of the stores group also rejected the deal, with 384 members voting against it. But the technical specialists group ratified a contract with 60 members voting in favor and 18 against.

“While the Mechanic & Related and Stores agreements were not approved, we expect to continue working toward new agreements and will look to the National Mediation Board for guidance on next steps with these work groups,” Missy Latham, American Airlines labor spokesperson, said in a statement.

Latham said the company is looking forward to reworking the agreements in a balanced way that will achieve ratification.

The union said the “no” votes amount to strike authorization. Airline strikes are rare. The Railway Labor Act, which safeguards interstate commerce, requires airline employees to be released from mediation before a strike is legal.

A presidential administration can also temporarily halt a strike once it has started.

Mechanics at Northwest Airlines went on strike in 2005 only to watch the airline operate normally with replacement labor. Northwest later restructured in bankruptcy and merged with Delta Air Lines.

The TWU unit at American Airlines has asked to be released from mediation by the National Mediation board.

Shares of AMR were down 4.02 percent at $5.98 on the New York Stock Exchange on Wednesday morning.