Washington, Jan 18 (AP) It is unclear who will take over at the Pentagon and the military services when the top leaders all step down Monday as President-elect Donald Trump is sworn into office.

As of Friday, officials said they had not yet heard who will become the acting defence secretary.

Officials said the military chiefs of the army, navy and air force were getting ready to step in as acting service secretaries -- a rare move -- because no civilians had been named or, in some cases, had turned down the opportunity.

As is customary, all current political appointees will step down as of noon EST on Inauguration Day (January 20), leaving hundreds of key defence posts open, including dozens that require Senate confirmation.

In addition to the top job and all three service secretaries, all of their deputies and senior policy staff will leave.

The Senate Armed Services Committee is expected to vote Monday on Trump's choice to head the defence department, Pete Hegseth, but the full Senate vote may not happen until days later. As a result, someone from the Biden administration will have to take over temporarily.

For the service secretaries, officials said that while things could still change before the inauguration, the Trump team is eyeing Gen Randy George, chief of staff of the army, to be that service's temporary head.

They said Gen David Allvin, chief of staff of the air force, and Adm Lisa Franchetti, the navy chief, are aware that they may have to step in if no civilian is named as acting secretary, and they are preparing for that possibility.

The officials, who spoke on condition of anonymity to discuss internal deliberations, said many senior Joe Biden administration leaders are reluctant to serve in the incoming Trump administration because they are concerned about policy changes they may be required to handle or enforce.

Usually, only people appointed by the president and confirmed by the Senate serve as a defence or service secretary, including in an acting capacity during a transition.

Trump could pull a confirmed member of the Biden administration from another agency and put that person at the Pentagon.

Civilian control of the military is a key tenet, but under the law the military chiefs of the services -- who are all Senate confirmed -- can take over on a temporary basis.

It's rare, but did happen more than 30 years ago.

Arnold Punaro, a retired Marine Corps Reserve two-star general, said that in 1993, Adm Frank Kelso, who was navy chief, was asked to serve as acting navy secretary when Bill Clinton became president because civilian leaders did not step up.

"It doesn't happen very often," said Punaro, who spent 14 years as a staff director on the Senate Armed Services Committee and has advised nominees through the confirmation process for decades.

"Normally you don't want the active-duty military serving in the civilian control positions. The practical reality is they are wearing both hats." The transition to a new secretary of defence has usually been an orderly process.

Four years ago, the deputy secretary of defence under Trump, David Norquist, became acting secretary for the two-day gap between the inauguration of Democratic President Joe Biden and the Senate vote to confirm Lloyd Austin as Pentagon chief.

President Barack Obama asked his Republican predecessor's defence secretary, Robert Gates, to stay on as his own Pentagon leader in 2009.

In 2017, Jim Mattis, Trump's pick to be secretary during his first term, was confirmed on Inauguration Day.

Various administrations have handled the handover differently. In many cases, people have been asked to stay on in a temporary role.

In one recent instance, officials said, the comptrollers of the services stepped in as acting secretaries because a key job in the coming months is to put together the massive, complex budget and more often the money people are considered less political.

This year's gap is further complicated by the fact that Trump and Hegseth have both pledged to rid the defence department of what they call "woke" generals -- or those who have supported diversity programmes.

That raises the possibility that even as the administration struggles to fill its political appointee slots, it may also be carving holes in the military leadership structure that will have to be filled.

When Senator Elissa Slotkin asked Hegseth during his nomination hearing if he intended to fire the current Joint Chiefs chairman, Gen CQ Brown, he answered, "Senator, every single senior officer will be reviewed based on meritocracy, standards, lethality, and commitment to lawful orders they will be given." Hegseth previously said that Brown should be fired.

Conservative groups have compiled lists of generals they believe should be fired for supporting diversity programmes. If Brown is fired, the vice-chairman would take over until a new chairman is confirmed. (AP) ARI