US Secretary of Defense Lloyd Austin has been admitted to the critical care unit as he undergoes treatment for a bladder issue, the Pentagon said late Sunday, in a statement citing his doctors.

"Tonight, after a series of tests and evaluations, the Secretary was admitted into the critical care unit at Walter Reed National Military Medical Center for supportive care and close monitoring," the statement read.

Austin's hospitalization comes weeks after it emerged that the 70-year-old had kept previous hospital stays secret and had not immediately informed US President Joe Biden of a cancer diagnosis, sparking criticism as the United States faces crises in the Middle East and Ukraine. 

Austin effectively vanished from the public eye for treatment for prostate cancer in December and again in January after suffering complications from the procedure.

This time, the public was alerted around two hours after he was sent to the hospital on Sunday afternoon, and Deputy Secretary of Defense Kathleen Hicks "assumed the functions and duties" of defense chief just before 5:00 pm (2200 GMT), according to Pentagon spokesman Pat Ryder.

"At this time, it is not clear how long Secretary Austin will remain hospitalized," added the latest statement, attributed to Dr John Maddox and Dr Gregory Chesnut of the Walter Reed National Military Medical Center in Maryland.

"The current bladder issue is not expected to change his anticipated full recovery. His cancer prognosis remains excellent."