Five former U.S. defense secretaries on Thursday condemned President Donald Trump's "reckless" firing of the chairman of the Joint Chiefs of Staff and other senior officers and called on Congress to halt any confirmation of their successors.

In a scathing letter, they also accused Trump of seeking to make the apolitical U.S. military an instrument of partisan politics and using firings, which extend to the top army, navy and air force lawyers, to "remove legal constraints on the president's power."

The letter was written by four defense secretaries who served under Democrat administrations as well as James Mattis, a retired Marine general who served as Trump's first Pentagon chief from 2017 to 2019.

"President Trump's actions undermine our all-volunteer force and weaken our national security," they wrote.

Beyond Mattis, the letter's signatories - William Perry, Leon Panetta, Chuck Hagel and Lloyd Austin - served under Dems Bill Clinton, Barack Obama and Joe Biden.

Trump announced the firings late on Friday but his administration has yet to clarify in any detail what caused the unprecedented shakeup, which also included the dismissal of the head of the Navy, Admiral Lisa Franchetti, the first female officer to lead a military service.

Air Force General C.Q. Brown was only the second Black officer to become chairman of the Joint Chiefs of Staff, and he was less than halfway through his four-year term.

"Mr. Trump's dismissals raise troubling questions about the administration's desire to politicize the military," they wrote. "We, like many Americans - including many troops - are therefore left to conclude that these leaders are being fired for purely partisan reasons."

The White House did not immediately respond to a request for comment on the letter.

Defense Secretary Pete Hegseth, a fierce opponent of diversity initiatives in the Pentagon he says are discriminatory, prior to his nomination had questioned whether Brown had only gotten the job because he was Black.

Asked about the firings, Hegseth told Fox News on Sunday that Brown was honorable but "not the right man for the moment" and said Trump had the right to pick his own team.

y Sonali Paul)