WASHINGTON — Amid tensions in the Middle East, Secretary of Defense Lloyd Austin was hospitalized, including spending four days in the intensive care unit, according to two senior administration officials.

The Pentagon did not inform senior officials in the White House’s National Security Council of Austin’s hospitalization until Thursday — three days after he arrived at Walter Reed Medical Center, a U.S. official confirms. Politico first reported the delay.

Austin released a statement Saturday taking responsibility for not disclosing his condition sooner.

“I am very glad to be on the mend and look forward to returning to the Pentagon soon. I also understand the media concerns about transparency and I recognize I could have done a better job ensuring the public was appropriately informed. I commit to doing better,” Austin said. “But this is important to say: this was my medical procedure, and I take full responsibility for my decisions about disclosure.”

Austin was not able to perform his duties since New Year’s Day, a senior defense official said Friday. Pentagon press secretary Maj. Gen. Pat Ryder told NBC News that Austin “resumed his full duties” on Friday evening.

Austin remained hospitalized Saturday. It was unclear when he would be released, the source said. Details about what ailed him were unavailable.

Austin was admitted to Walter Reed National Military Medical Center on Monday night for “complications following a recent elective medical procedure,” Ryder said in a statement Friday evening.

Pentagon officials will not disclose what the procedure was or when it occurred. They also refuse to provide any details about how serious Austin’s condition was this week.

Deputy Secretary of Defense Kathleen Hicks, who was on leave, has picked up his duties, the senior defense official said.

Hicks was on pre-scheduled leave when Austin was hospitalized this week, according to a defense official. She has been on vacation in Puerto Rico, the official said.

She “has maintained full communication with the DOD staff throughout,” the official said. “She has monitored DOD’s day-to-day operations and conducted some routine business.”

The third in line among Pentagon civilian leadership, the office of the under secretary of defense for policy, is occupied temporarily as an acting position because nominee Derek Chollet has not been confirmed by the U.S. Senate, the source said. Sasha Baker has been acting in the role since last summer.

“At all times, the Deputy Secretary of Defense was prepared to act for and exercise the powers of the Secretary, if required,” Ryder said in the statement.

Pentagon officials refused to describe Austin’s condition or say what exactly prevented him from carrying out his duties this week.

The Pentagon also would not release details about the procedure or when it took place, and it won’t say whether Austin was taken to the hospital in an ambulance.

Asked why the Pentagon kept Austin’s hospitalization secret, Ryder told NBC News, “This has been an evolving situation in which we had to consider a number of factors.”

Those include Austin’s personal privacy, he said.

While Austin has been hospitalized, the U.S. conducted a rare and controversial strike against a senior Iranian-backed militia member in Baghdad, bases with Americans have been attacked at least six times, and the Biden administration has been considering options to strike Houthi militants in response to their continued attacks against ships in the Red Sea.

Tension in the region centers on the Israel-Hamas war. In October, the U.S. Navy sent two carrier strike groups to the eastern Mediterranean Sea in response.

Iran-backed Houthi militants based in Yemen, unhappy with Israel’s attacks on Palestinian communities, have been attacking container ships in the Red Sea.

On Dec. 30, a U.S. Navy destroyer shot down two Houthi missiles after it responded to a strike on a container ship that resulted in no injuries or damage, military officials said at the time. The next day, Houthi rebels fired on Navy helicopters responding to a container ship distress call. The Navy fired back, sinking three small boats and killing the crews, officials said.

The U.S. is also wary of further regional entanglement as Iran-backed Hezbollah militants in Lebanon exchange munitions with Israel along its northern border.