Austin was admitted to Walter Reed National Military Medical Center on January 1 due to complications from an elective medical procedure, but the Pentagon did not make any public announcement until four days later, and also waited to notify other top government figures.

The 70-year-old secretary's hospitalization comes with Washington struggling to contain the fallout from the Israel-Hamas war, which has sparked violence against American forces in Iraq and Syria as well as attacks on international shipping.

With the Middle East in turmoil, the idea that "for four days the secretary of defense is in a hospital and (President Joe) Biden doesn't know is shocking," Ian Bremmer, the president of the Eurasia Group political risk firm, said Monday.

Bremmer said the situation gives the president an opportunity to replace Austin, but the White House has stood by the secretary.

Austin underwent an unspecified medical procedure on December 22 and was discharged the following day, but began experiencing "severe pain" on January 1 and was taken by ambulance to Walter Reed, Pentagon spokesman Major General Pat Ryder told journalists on Monday.

Some of Austin's authorities were transferred to Deputy Defense Secretary Kathleen Hicks on January 2, but she was not told that he was hospitalized until two days later, Ryder said.

- Criticism from Congress -

The US National Security Advisor Jake Sullivan was likewise informed on January 4, bringing the White House into the loop, while Congress was not told until the day after that -- the same day the Pentagon made a public announcement.

Ryder said Austin's chief of staff "had been out sick with the flu, which caused a delay in these notifications."

"We are currently reviewing how we can improve these notification procedures, to include White House and congressional notifications," he said.

Ryder also said he was informed of Austin's hospitalization on January 2, but "did not feel that I was at liberty" to disclose information on the secretary's condition "until we knew more."

The lack of notification has drawn criticism from Congress, with some Republican lawmakers calling on Austin to go.

"It is shocking and absolutely unacceptable that the Department of Defense waited multiple days to notify the president, the National Security Council, and the American people," Representative Elise Stefanik said in a statement, calling for Austin's "immediate resignation."

Former president Donald Trump also weighed in, saying in a social media post that Austin "should be fired immediately for improper professional conduct and dereliction of duty."

But the White House has backed him, with Press Secretary Karine Jean-Pierre saying, "The president has complete confidence, continues to have confidence in Secretary Austin."

Ryder also said Austin -- who remains hospitalized but is no longer in intensive care and has resumed his full duties -- "has no plans to resign."

"Nothing is more important to the secretary of defense and the (Defense) Department than the trust and confidence of the American public we serve," Ryder said, adding that "we will continue to work hard every day to earn and deserve that trust."

Austin meanwhile said in a statement on Saturday that he took "full responsibility for my decisions about disclosure," and admitted that he "could have done a better job ensuring the public was appropriately informed."