U.S. Defense Secretary Lloyd Austin has been hospitalized since Monday for an unspecified medical matter, the Pentagon said late Friday, without detailing why he was being treated or why it kept his hospital stay secret all week.

Austin, who is 70, sits just below President Joe Biden at the top of the chain of command of the U.S. military and his duties require him being available at a moment's notice to respond to any manner of national security crisis.

The Pentagon did not say whether Austin ever lost consciousness before or after he was admitted to Walter Reed National Military Medical Center on Jan. 1, or the extent to which his duties were assumed by his deputy, Kathleen Hicks.