Deputy Secretary of Defense Kathleen Hicks was not notified until two days later that Austin had been admitted on New Year's Day to Walter Reed National Military Medical Center, CNN reported on Sunday citing two unidentified defense officials.

Reuters reported on Saturday that President Joe Biden was only informed of the hospitalization on Thursday evening, according to a U.S. official, speaking on condition of anonymity. Still, Biden maintained confidence in Austin and the two spoke on Saturday evening, a second U.S. official said.

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

Politico reported that General Charles Q. Brown, chairman of the Joint Chiefs of Staff, did not learn of Austin's hospitalization until Tuesday.

Austin, who is 70, said in a statement on Saturday that he took "full responsibility" for the secrecy surrounding an ongoing, week-long hospitalization for a still unspecified medical condition.

CBS reported Sunday that Austin remains in the hospital.

A spokesperson for the Pentagon did not immediately reply to a request for comment on when Hicks and Brown were notified of Austin's hospitalization or if he is still at Walter Reed.

Secretary of State Anthony Blinken said during a press conference on Sunday that he "wasn't aware of his medical issue" and had spoken to Austin last weekend.

"I'm very much looking forward to seeing him fully recovered and working side by side," Blinken said.

It remains unclear the extent to which his duties were delegated to Hicks, or whether Austin was involved in any key decisions during his absence.

The Pentagon has yet to detail why Austin is being treated, whether he lost consciousness over the past week or offer any details on when he might be discharged from the hospital.

Democratic Representative James Clyburn told CNN on Sunday that he had been told Austin is "now in charge of things as he was before the illness" and that he didn't think the lack of disclosure was a dereliction of duty.

"He does have a duty to keep the public informed, and I don't know whether it was him or somebody inside of the military establishment that decided to do it this way, but I'm sure he will do a little better going forward, as he said he would," Clyburn said.

(Reporting by Hannah Lang; Additional reporting by Susan Heavey in Washington; Editing by Caitlin Webber and Sandra Maler)

By Hannah Lang