Austin, who is 70, was admitted on New Year's Day to Walter Reed National Military Medical Center for what the Pentagon has said were "complications following a recent elective medical procedure," a fact the Defense Department kept under wraps for five days.

The top Republican on the Senate Armed Services Committee, Senator Roger Wicker, accused the Pentagon of failing to inform Congress immediately about such matters, as required by law.

But it is unclear how widely the information was shared even within President Joe Biden's administration. A U.S. official, speaking on condition of anonymity, said Biden was only informed on Thursday evening. Still, Biden maintained confidence in Austin and the two spoke on Saturday evening, a second U.S. official said.

Austin sits just below Biden at the top of the chain of command of the U.S. military and his duties require his being available at a moment's notice to respond to any manner of national security crisis.

It remains unclear the extent to which his duties were delegated to his deputy, Kathleen Hicks, or whether Austin was involved in any key decisions during his absence.

The Pentagon has yet to detail why Austin is being treated, whether he lost consciousness over the past week or offer details on when he might be discharged from the hospital.

"I recognize I could have done a better job ensuring the public was appropriately informed. I commit to doing better," Austin said in a written statement.

"But this is important to say: this was my medical procedure, and I take full responsibility for my decisions about disclosure."

Wicker said the episode further eroded the public's trust in the Biden administration, citing past failures to quickly disclose information about national security incidents, including the appearance of a Chinese spy balloon over the United States last year.

"When one of the country's two National Command Authorities is unable to perform their duties, military families, Members of Congress, and the American public deserve to know the full extent of the circumstances," Wicker said in a statement.

A spokesperson said on Saturday that Austin resumed his full duties on Friday evening but remained in the hospital.

The Pentagon Press Association, in a letter to Pentagon officials on Friday evening, criticized the Defense Department's secrecy, saying Austin was a public figure who had no claim to medical privacy in such a situation.

It also noted that even U.S. presidents disclose when they must delegate duties due to medical procedures.

"At a time when there are growing threats to U.S. military service members in the Middle East and the U.S. is playing key national security roles in the wars in Israel and Ukraine, it is particularly critical for the American public to be informed about the health status and decision-making ability of its top defense leader," it wrote.

Reuters correspondent Phil Stewart is a member of the association's board of directors.

Military Reporters and Editors (MRE), a non-profit organization for journalists covering the U.S. military, said the decision to only release the information on a Friday evening, when online readership is typically lower, "is keeping in the worst traditions of obfuscation and opacity."

"This is a violation of the intent and spirit of the Pentagon's own Principles of Information, and it fails to meet the standards of public disclosure for senior government officials unable to exercise their duties," MRE wrote in a statement.