Austin kept President Joe Biden and Congress in the dark about his prostate cancer diagnosis for weeks and did not inform them about his hospitalization at the beginning of the year, leading to growing calls for him to resign or be fired.

The situation is a major headache for Biden in an election year, putting the president on the defensive and providing an opening for attacks by Republicans who are already painting him as too old to manage the job.

"The objective of the review is to examine the roles, processes, procedures, responsibilities, and actions related to the Secretary of Defense's hospitalization in December 2023 - January 2024," the memo said, referring to Austin's prostate cancer treatment and subsequent complications.

The probe will also assess whether Defense Department "policies and procedures are sufficient to ensure timely and appropriate notifications and the effective transition of authorities," it added.

Austin, a 70-year-old career soldier, underwent minor surgery to treat the cancer on December 22, returning home the following day, but was readmitted due to complications including nausea and severe pain on January 1.

- Undisclosed cancer diagnosis -

The White House was not informed about Austin's hospitalization until last Thursday, while Congress was not told until Friday, and Biden did not learn of the cancer diagnosis until this week.

In response, White House chief of staff Jeff Zients ordered an urgent review of the rules for when senior US officials are incapacitated, as did Austin's chief of staff Kelly Magsamen.

The secretary's undisclosed hospitalization left a key national security official unaccounted for at a time when Washington's forces are frequently under fire in Iraq and Syria, and Yemeni rebels are attacking international shipping in the Red Sea.

Republican lawmakers have called for Austin to go and threatening him with impeachment -- the latest Biden administration official they are targeting in a bid to hammer the Democrats ahead of November's election.

Various Democrats have also expressed concern, but only one member of Biden's party has called for the defense secretary to leave office.

Unlike the endless "you're fired" of Donald Trump's turbulent administration, Biden has repeatedly been unwilling to sack senior officials.

He notably clung to National Security Advisor Jake Sullivan after the chaotic US exit from the Afghanistan in 2021, and has likewise stood by Austin so far.