"He was informed by the chief of staff earlier this morning," National Security Council spokesman John Kirby told a briefing.

Austin was diagnosed early December and underwent surgery on December 22, before being hospitalized with complications on January 1, the Pentagon said earlier Tuesday.

The White House was not told that the defense chief was in hospital until January 4, at a time of high tensions in the Middle East and Ukraine.

The White House insisted that Biden, who is seeking a second term in office in elections in November, was in control of his government despite not knowing until Tuesday about the cancer diagnosis.

"It is not optimal for a situation like this to go as long as it did without the commander-in-chief knowing about it or the national security adviser knowing about it, or frankly other leaders at the Department of Defense," Kirby said during a briefing at the White House.

"It's not the way this is supposed to happen... It's not good. We want to make sure that it doesn't happen again."

Biden ordered a series of military actions during that period including a strike on an Iraqi militia leader, while US forces had successfully shot down missiles fired by Yemen's Houthi rebels in the Red Sea.

"He has been on on on top of these issues, all the way throughout," Kirby said.

The president had "complete confidence" in Austin and was looking forward to having him back at the Pentagon, the spokesman added.

White House chief of staff Jeff Zients earlier Tuesday ordered an urgent review Tuesday of the rules for when senior US officials are incapacitated amid the Austin row, according to a memo obtained by AFP.

"There's an expectation that if a cabinet officer becomes hospitalized, and for whatever reason can't continue to perform the duties even temporarily, that that will be notified up the chain of command to the commander in chief," Kirby added.