The Biden administration was not told for days that US Defence Secretary Lloyd Austin had been hospitalised, at least one official has told US media.

Mr Austin, 70, was admitted to the Walter Reed Medical Center on Monday due to complications following surgery.

An official told the BBC's US partner CBS that the White House was not informed of this until at least Thursday morning.

Mr Austin has accepted responsibility for the lack of communication.

"I recognise I could have done a better job ensuring the public was appropriately informed," he said in a statement.

"I commit to doing better."

The defence secretary sits just below the president in the chain of command for the US military.

Mr Austin added that it was "important to say: this was my medical procedure, and I take full responsibility for my decisions about disclosure".

"I am very glad to be on the mend and look forward to returning to the Pentagon soon."

He is thought to still be in hospital but a defence department spokesperson quoted by the AFP news agency said he resumed his full duties on Friday.

It is not clear how many of his responsibilities Mr Austin has been able to carry out due to his illness, nor the extent to which Deputy Secretary of Defence Kathleen Hicks stepped in to help.

Officials told CNN that she had periodically taken on his duties during his time in hospital, while she herself was in Puerto Rico on holiday.

Meanwhile, Joe Biden is reported to have had a "warm" conversation with Mr Austin on Saturday.

"The president has full confidence in Secretary Austin. He's looking forward to him being back at the Pentagon," one official told AFP.

Republicans are among those to have expressed serious concern over the situation.

"The secretary of defence is the key link in the chain of command between the president and the uniformed military, including the nuclear chain of command, when the weightiest of decisions must be made in minutes," Senator Tom Cotton in a statement.

"If this report is true, there must be consequences for this shocking breakdown."

Fellow Senator Roger Wicker said that while he was pleased to hear of Mr Austin's recovery, "the fact remains that the Department of Defence deliberately withheld the secretary of defence's medical condition for days. That is unacceptable."

The Pentagon Press Association, made up of journalists who cover the defence department, also criticised the apparent lack of transparency in a letter to the Pentagon on Friday.

"At a time when there are growing threats to US military service members in the Middle East and the US is playing key national security roles in the wars in Israel and Ukraine, it is particularly critical for the American public to be informed about the health status and decision-making ability of its top defence leader," it said.

Mr Austin, a retired four-star general, became the first African-American defence secretary in 2020.