Washington DC - US Defense Secretary Lloyd Austin on Saturday accepted responsibility for failing to disclose a recent hospitalization, following reports that even top White House officials and President Joe Biden were in the dark that he was ill and unable to carry out his duties.

The Pentagon waited until Friday evening to announce that Austin had been hospitalized four days prior "for complications following a recent elective medical procedure" – a breach of standard protocol at a time when the US is embroiled in the Middle East crisis.

NBC News reported the 70-year-old was in the intensive care unit for four days.

He remained at Walter Reed National Military Medical Center on Saturday and a discharge date was not yet known, a Defense Department spokesperson said.

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"I recognize I could have done a better job ensuring the public was appropriately informed. I commit to doing better," Austin said in a statement. "But this is important to say: this was my medical procedure, and I take full responsibility for my decisions about disclosure."

He added that he would be "returning to the Pentagon soon," thanking doctors and staff at Walter Reed for their care.

Austin resumed "full duties" Friday evening, the Defense Department spokesperson said.

Austin's hospitalization comes with tensions soaring in the Middle East as a result of the Israel-Gaza war, with Houthi rebels in Yemen striking shipping lanes while forces in Iraq and Syria are attacking US troops with rockets and drones.

Deputy Secretary of Defense Kathleen Hicks was prepared to step in during Austin's illness, and made some routine decisions on his behalf during the time, a Defense Department spokesperson told AFP.

Politico was the first of several media outlets to report that Austin had been in the hospital for three days before Pentagon officials told National Security Adviser Jake Sullivan and other top White House officials.

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Sullivan then informed Biden, the outlet said. It also reported that Congress found out about Austin's hospitalization 15 minutes before Friday evening's public statement.

A White House official on Saturday did not discuss the timing of when Biden was informed of Austin's absence but said the two men "spoke this evening and it was a warm conversation."

"The President has full confidence in Secretary Austin. He's looking forward to him being back at the Pentagon," the official told AFP.

Arkansas Senator Tom Cotton, a Republican member of the Armed Services Committee, demanded an explanation.

"The Secretary of Defense is the key link in the chain of command between the president and the uniformed military, including the nuclear chain of command, when the weightiest of decisions must be made in minutes. If this report is true, there must be consequences for this shocking breakdown," he said in a statement Saturday.

The Pentagon Press Association, comprised of journalists who report on the Defense Department, expressed "significant concerns" over the apparent cover-up.

"The public has a right to know when US Cabinet members are hospitalized, under anesthesia or when duties are delegated as a result of any medical procedure," the group said in a letter to the Pentagon press secretary.