(Reuters) -U.S. President Joe Biden said on Thursday he is concerned that Boeing has locked out its unionized firefighters, raising pressure on the U.S. planemaker to resolve the contract dispute.

Earlier this month, Boeing locked out nearly 130 members of the International Association of Fire Fighters Local I-66 who have rejected two contract offers.

"I'm concerned by reports that Boeing locked out IAFF I-66 members," Biden wrote on the social media site X. "I encourage folks to return to the table to secure a deal that benefits Boeing and gets these firefighters the pay and benefits they deserve."

Biden's message in support of the firefighters raises the possibility of White House involvement in separate talks between Boeing and its unionized factory workers in Washington state, two sources familiar with the matter said. Those workers produce the planemaker's best-selling jet.

"We appreciate the support from U.S. President Joe Biden and think it's important that he supports collective bargaining," said Jon Holden, president of the International Association of Machinists and Aerospace Workers District 751 local, which represents the Seattle-area workers.

"Our union strongly believes that collective bargaining is the process for reaching an agreement."

The International Association of Machinists and Aerospace Workers (IAM) is negotiating a new contract on behalf of over 30,000 workers who build Boeing's 737 MAX jets, at a time when the planemaker needs to ramp up production.

Boeing 737 MAX jetliner production has fallen sharply as U.S. regulators step up factory checks following a blowout on a nearly new 737 MAX 9 in January, blamed on an assembly error.

Biden, who has racked up union endorsements as he runs for re-election in November against former President Donald Trump, has visited picket lines for the United Auto Workers (UAW). He considers support for labor to be a cornerstone of his economic policies.

The votes of union members could be crucial to deciding the 2024 election in key swing states like Michigan, Pennsylvania, Nevada and Wisconsin.

"Imagine if he'd throw his hat in for the IAM," said one investor who spoke on condition of anonymity.

Boeing has said its latest offer, which would increase firefighters' take-home pay from $91,000 on average to $112,000 on average in the first year of the contract, remains on the table.

"We remain committed to securing an agreement. Our offer provides significant pay increases and increased benefits," Boeing said in a statement. "The union should allow our employees to vote (on) our offer, which was presented before the lockout."

IAFF General President Edward Kelly said the firefighters are "are grateful that the President stands up for collective bargaining" and continues to have their backs.