A Southwest Airlines Co. plane takes off as representatives and pilots from the Southwest Airlines Pilots’ Association (SWAPA) demonstrate outside Chicago Midway International Airport (MDW) in Chicago, Illinois, U.S., on Wednesday, May 18, 2016.Daniel Acker / Bloomberg / Getty ImagesSouthwest Airlines pilots’ union said Thursday it sought to be released from federal mediation for a new labor contract, laying the groundwork for a potential strike as talks with the carrier haven’t yet yielded an agreement.The airline and union, the Southwest Airlines Pilots Association, have been in contract talks for more than three years and negotiations have been tense. The Dallas-based carrier’s pilots voted to authorize the union to call a possible strike last month, a poll that the union called on the heels of a holiday meltdown at the end of last year.”Regrettably, I must inform you that SWAPA and Southwest have been unable to meaningfully resolve numerous important, outstanding issues, and that further mediation will likely not result in any additional agreements between the parties,” Jody R. Reven, the negotiating committee’s chairman, wrote to the National Mediation Board on Thursday, according to a letter seen by CNBC.The union said Southwest has refused to engage “in substantive discussions or offer ratifiable proposals” on issues like better pay, work rules, quality-of-life improvements and fatigue mitigation, according to a letter the union sent to the National Mediation Board.Southwest’s vice president, labor relations, Adam Carlisle, said in a statement that the company disagrees with the need to be released from mediation.”We’ve continued meeting regularly with SWAPA and, in fact, made an industry-leading compensation proposal and scheduling adjustments to address workplace quality-of-life issues for our Pilots,” he said. “We feel confident that mediation will continue driving us even closer to a final agreement that will benefit both our Pilots and Southwest Airlines.”Pilot strikes in the U.S. are extremely rare, and the Southwest Airlines Pilots Association’s request does not mean that one is imminent because of procedures in U.S. labor law. The last major U.S. passenger airline strike in the country was at Spirit Airlines in 2010.There are several so-called cooling off periods should the National Mediation Board declare an impasse between Southwest and its pilots’ union. Those last 30 days apiece, giving time for a potential agreement.