By Valerie Volcovici and David Shepardson

WASHINGTON (Reuters) - The Federal Aviation Administration fired fewer than 400 employees out of its workforce of 45,000, Transportation Secretary Sean Duffy said on Monday, as questions rise around air traffic safety amid a spate of recent plane accidents.

Duffy posted the number of layoffs in a social media post message on X, formerly known as Twitter, responding to a post by his Democratic predecessor Pete Buttigieg, who has been critical of the Trump administration's Department of Transportation.

"Less than 400 were let go, and they were all probationary, meaning they had been hired less than a year ago. Zero air traffic controllers and critical safety personnel were let go," Duffy wrote on Monday.

The Trump administration sent air traffic controllers buyout offers but later said they were not eligible, also declaring other safety officials, including TSA officers, ineligible. The FAA remains about 3,500 controllers short of targeted staffing levels.

The Professional Aviation Safety Specialists union said Saturday the FAA had fired several hundred probationary FAA employees.

The union said Monday it believed just under 300 FAA workers it represents were fired, including maintenance mechanics, aeronautical information specialists, aviation safety assistants and management and program assistants.

"These are positions that are vital to supporting public safety," a union spokeswoman said.

The FAA and Transportation Department have declined to say what jobs the fired workers held or why they were fired.

The disclosure came on the same day that Elon Musk's government downsizing team DOGE was visiting the FAA's Air Traffic Control command center in Warrenton, Virginia, Duffy said.

Senator Maria Cantwell, the top Democrat on the Senate Commerce Committee, on Monday slammed the Trump administration for firing FAA employees who inspect and maintain air traffic control communications, radio and computer systems - especially after a series of fatal crashes.

“The FAA is already short 800 technicians and these firings inject unnecessary risk into the airspace — in the aftermath of four deadly crashes in the last month," she said.

USDOT said Monday FAA "continues to hire and onboard air traffic controllers and safety professionals, including mechanics and others who support them."

Duffy said on X that the Trump administration plans to overhaul what he called "our outdated, World War II-era air traffic control system."

(Reporting by Valerie Volcovici and David Shepardson. Editing by Gerry Doyle)