Austin was admitted to Walter Reed National Military Medical Center on the evening of January 1 "for complications following a recent elective medical procedure," Pentagon Press Secretary Major General Pat Ryder said in a statement, without providing details on the complications or the duration of the hospital stay.

"He is recovering well and is expecting to resume his full duties today," Ryder said.

The Pentagon had not previously announced that Austin was hospitalized.

The secretary's hospitalization comes with tensions soaring in the Middle East as a result of the Israel-Hamas war, with Iran-backed forces in Yemen striking shipping lanes while others in Iraq and Syria are attacking US troops with rockets and drones.

On Thursday, the United States carried out a strike that killed an Iran-backed commander in Baghdad whom the military accused of involvement in attacks on American forces, infuriating the Iraqi government.

"At all times, the deputy secretary of defense (Kathleen Hicks) was prepared to act for and exercise the powers of the secretary, if required," Ryder said.

A Defense Department spokesperson later told AFP that the deputy secretary "is automatically authorized to perform the duties of the secretary if he is unable to perform them," and that "she did make some routine decisions on his behalf this week."

Austin was aware of the Baghdad strike, the spokesperson said, adding that the secretary and US President Joe Biden had previously authorized the action.