WASHINGTON (Reuters) -The CEOs of the five largest U.S. airlines on Thursday plan to back the Trump administration's proposal to spend tens of billions of dollars to reform the aging air traffic control system and boost hiring, officials said.

Delta Air Lines, United Airlines, American Airlines, Southwest Airlines and JetBlue Airways chief executives are all expected to speak at an event Thursday in which Transportation Secretary Sean Duffy will detail his proposal to Congress. Other aviation industry companies will also attend.

Also expected to attend are some relatives of the 67 people killed in the January 29 mid-air collision of an American Airlines regional jet and a U.S. Army helicopter near Reagan Washington airport.

Airlines and others may also call for around $30 billion or more in total funding, but Duffy is not expected to release a specific figure beyond the "tens of billions" he said in March the administration would need pending further discussions with lawmakers.

Major unions representing aviation pilots, air traffic controllers and flight attendant groups and other aviation associations are also taking part in the event.

Congress is also considering $12.5 billion in initial funding as part of bill under consideration.

"We're going to build a brand new air traffic control system, from new telecom to new radars to new infrastructure," Duffy said earlier this week, estimating it will take three to four years to address.

Trade group Airlines for America, which will also take part in the event Thursday, praised Duffy Wednesday for "his commitment to building a new improved air traffic control system."

Delta and United both confirmed their CEOs will attend. A Delta spokesperson said CEO Ed Bastian "fully supports" Duffy's plan and says this is "a once in a lifetime opportunity to address these longstanding challenges."

Duffy also wants funding to speed air traffic control hiring. The FAA, which said in March it planned to hire 2,000 air traffic controller trainees this year, said it will offer new bonuses and payments for new controllers and to convince retirement-eligible controllers to stay on the job.

A persistent shortage of controllers has delayed flights and many are working mandatory overtime and six-day weeks. The FAA is about 3,500 air traffic controllers short of targeted staffing levels.