American Airlines Group Inc.’s unionized flight attendants are threatening to strike after the latest round of talks with the company concluded without an agreement on a new contract.

“All American Airlines flight attendants should prepare for an upcoming strike,” the union said Thursday in an email. It added that “we remain apart on the key issues,” such as the date of a signing wage increase and retroactive pay.

The union said it didn’t believe that further negotiations will be scheduled by the National Mediation Board, which has overseen talks by the two parties. The union is restricted from walking off the job while in mediation. If the board declares talks at an impasse, it would trigger a 30-day “cooling off” period prior to a possible strike.

American Airlines said it expects the NMB to schedule additional dates for negotiations, saying a deal is “within reach.”

“We made good progress in negotiations this week, adding even more to the industry-leading proposal we’ve had on the table for months,” the company said in a statement. “We look forward to continuing negotiations so our flight attendants can benefit from the contract they deserve.”

This isn’t the first time the union broached the subject of a walkout. Following a round of negotiations last month, the union told members in a message on May 31 to begin preparing for a strike.