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 spokesman Major General Pat Ryder said in a statement, without providing details on the nature of the complications or the duration of the hospital stay.

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

Austin's hospitalization comes at a time of high tensions in the Middle East that are linked to the Israel-Hamas war, with Iran-backed forces in Yemen striking shipping 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 was prepared to act for and exercise the powers of the secretary, if required," Ryder said, without specifying if that had been necessary.