The US defence secretary Lloyd Austin’s hospitalisation remained secret for longer than previously known, officials disclosed on Sunday, with his deputy on a long list of people up to President Joe Biden who were in the dark for days.

The Pentagon released new details on Sunday about Austin’s continued hospitalisation, saying he had an initial medical procedure as far back as 22 December from which he went home a day later.

Then on Monday 1 January, Austin, who is 70, had to be admitted to intensive care at Walter Reed military hospital in Maryland after experiencing severe pain.

Deputy defence secretary Kathleen Hicks assumed some of his operational duties on Tuesday 2 January, while she was on vacation in Puerto Rico, a US official said. But Hicks was not told why Austin was unavailable until Thursday 4 January, three officials told Reuters. That is the same day that Biden and other senior White House officials were told that Austin had been hospitalised since Monday 1 January, five officials said.

Hicks was in Puerto Rico on leave but had communications equipment with her to remain in contact and had already been tasked with some secretary-level duties by Tuesday 2 January.

Austin sits just below Biden at the top of the chain of command of the US military, and his duties require him being available at a moment’s notice to respond to any national security crisis. That includes always being ready to enter secure communications with other officials in the event of an incoming nuclear attack – something that would be difficult to do from an ICU bed.

At least some circles in the Pentagon were aware of Austin’s situation earlier. The chairman of the joint chiefs of staff, Gen Charles “CQ” Brown, learned of the hospitalisation on Tuesday 2 January, one US official said.

The Pentagon’s failure to disclose Austin’s hospitalisation for days, including to Biden, the National Security Council and top Pentagon leaders, reflects a stunning lack of transparency about his illness, how serious it was and when he may be released. Such secrecy, when the US is juggling myriad national security crises, runs counter to normal practice with regards to notifying the president and other senior US officials and cabinet members.

The Pentagon said on Sunday that the delay in disclosing his whereabouts was at least partially due to Austin’s chief of staff being off sick. Austin said in a statement on Saturday that he took “full responsibility” for the secrecy surrounding his hospitalisation.

On Sunday, a Pentagon statement released by Maj Gen Pat Ryder did not provide any details about the medical procedure or what actually happened to require Austin to be in intensive care at Walter Reed.

Ryan said Austin was placed in the hospital’s intensive care unit “due to his medical needs, but then remained in that location in part due to hospital space considerations and privacy”.

The Pentagon did not say if Hicks was given an explanation on Tuesday for why she was assuming some of Austin’s duties, but temporary transfers of authority are not unusual and are often done without detailed explanation. Hicks decided not to return from Puerto Rico after she was informed that Austin would resume full control on Friday.

The US Congress was only informed on Friday, shortly before the Pentagon published a statement.

The US secretary of state, Antony Blinken, said during a press conference on Sunday that he “wasn’t aware of [Austin’s] medical issue”.

“I’m very much looking forward to seeing him fully recovered and working side by side,” Blinken said.

Several lawmakers have said the communication breakdown raises serious questions about Pentagon leadership.

A joint statement from the top Democrat and Republican lawmakers on the House armed services committee called for the Pentagon to be transparent about Austin’s health “and the decision-making process that occurred in the past week”.

“Several questions remain unanswered including what the medical procedure and resulting complications were, what the secretary’s current health status is, how and when the delegation of the secretary’s responsibilities were made, and the reason for the delay in notification to the president and Congress.”

The top Republican on the Senate armed services committee, Roger Wicker, also took aim at Austin’s handling of his hospital stay. “When one of the country’s two national command authorities is unable to perform their duties, military families, members of Congress, and the American public deserve to know the full extent of the circumstances.”

The Pentagon said Austin remained in hospital on Sunday, and was recovering well and in good spirits. “Since resuming his duties on Friday evening, the secretary has received operational updates and has provided necessary guidance to his team.”

With Reuters and Associated Press