(Reuters) - Maintenance workers at United Airlines UAL.N overwhelmingly rejected a new contract offer, their union said Tuesday, adding it would seek approval from U.S. regulators to strike.

United Airlines planes are seen on platform at the Newark Liberty International Airport in New Jersey, July 8, 2015. REUTERS/Eduardo Munoz

More than 93 percent of mechanics who voted on the offer decided against it, the International Brotherhood of Teamsters said in a statement.

The union said it would petition the U.S. National Mediation Board to release it from mediated talks, although it will face a number of hurdles before workers can receive the legal go-ahead for a walkout. The U.S. agency did not immediately return requests for comment.

The vote marked a setback in United’s years-long effort to reach a joint deal with the technicians. It has yet to conclude a contract covering maintenance workers from both United and Continental since the carriers merged in 2010.

“At a time when United Airlines is incredibly profitable, it is clear that mechanics deserve a better offer from the company,” Teamsters President Jim Hoffa said in the statement.

Chicago-based United more than doubled its adjusted fourth-quarter profit from a year ago to $934 million. Its contract offer provided for an immediate 25 percent raise and bonus checks averaging $9,000, although the Teamsters expressed concern regarding wages for incoming mechanics.

“Although we are disappointed by this outcome, we are eager to get right back to the table,” Oscar Munoz, chief executive of parent United Continental Holdings Inc, said in a post on a public company website that is focused on labor talks.

Munoz has made boosting workers’ morale a priority since taking on the airline’s top job in September.

The National Mediation Board is not obliged to accept the Teamsters’ request to halt talks, said Jerry Glass, president of labor relations consultancy F&H Solutions Group.

If it does, United and the union will have 30 days to reach a deal, after which the U.S. executive branch or Congress can intervene to avert a strike, Glass said.

There is “a very high likelihood” based on precedent that the U.S. president would appoint an emergency board to recommend a solution to prevent disruptions to passengers and interstate commerce, he said.

United is the third-largest U.S. airline based on January flight capacity.

United Airlines said on Tuesday it reached a tentative agreement with the Professional Airline Flight Control Association that extends the contract of more than 420 dispatchers through the end of 2021.