FedEx Corp. pilots are inching closer to a potential strike as negotiations stall on a new labor agreement.

Leaders of the FedEx Express pilots union approved a strike authorization vote, according to a statement Wednesday from the Air Line Pilots Association. A date for the vote wasn’t specified. The union said no new talks have been scheduled between the two sides.

The move underscores the rift between the union and FedEx management despite almost six months of oversight by a federal mediator. Calling for a vote is one step in a lengthy process laid out under federal law that could lead to an eventual strike. 

The potential vote has no impact on FedEx’s service and the company said in a statement that it’s “still in productive negotiations” with its pilots.

“Strike authorization votes are a common tactic for labor organizations,” FedEx said. Shares of the Memphis, Tennessee-based company fell 1.4% in New York trading.

The FedEx Express talks are occurring at the same time the four largest US passenger airlines engage in efforts to reach new pilot contracts. Relations across the industry have been tense as unions seek increases in compensation and more flexible work schedules, and pilots have frequently picketed outside of airports and sites of company investor meetings.