Austin kept Biden and lawmakers in the dark about being diagnosed with prostate cancer for weeks, and did not inform them for days about his hospitalization on January 1 for complications from his treatment.

While the situation has been a headache for Biden, putting him on the defense and providing an opening for Republican attacks in an election year, the president has resisted pressure to part ways with the secretary.

"I'm grateful for the excellent care I received at Walter Reed National Military Medical Center and want to thank the outstanding doctors and nursing staff for their professionalism and superb support," Austin said in a statement.

"As I continue to recuperate and perform my duties from home, I'm eager to fully recover and return as quickly as possible to the Pentagon," he said.

Austin, a 70-year-old career soldier, underwent minor surgery to treat the cancer on December 22, returning home the following day.

But he was readmitted due to complications including nausea and severe pain on January 1.

- White House, Pentagon reviews -

The White House was not informed about Austin's hospitalization until January 4, while Congress was not told until the following day, and Biden did not learn of the cancer diagnosis until January 9.

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 Pentagon's independent inspector general also announced a review "to examine the roles, processes, procedures, responsibilities, and actions related to the secretary of defense's hospitalization" in both December and January.

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 Yemen's Huthi rebels are attacking international shipping in the Red Sea.

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

But unlike the endless "you're fired" of Donald Trump's turbulent administration, Biden has emphasized stability, sticking to his close advisors through various crises, including the traumatic final withdrawal of US troops from Afghanistan in 2021.

The president said Friday he remained confident in Austin, though he lamented the Pentagon chief's lapse in judgment in failing to inform him about the hospitalization.