US Defence Secretary Lloyd Austin has transferred duties to his deputy while in hospital for an "emergent bladder issue", the Pentagon has said.
Mr Austin, 70, is in the critical care unit at the Walter Reed National Military Medical Center in Virginia.
Officials said it is so far unclear how long he will remain in hospital.
The defence secretary was under scrutiny for failing to disclose his prostate cancer diagnosis and hospital stays in December and January.
On Sunday afternoon, the Pentagon released a statement saying Mr Austin had been taken to Walter Reed for treatment, adding that the White House and senior defence officials had been notified.
Hours after, at about 16:55 local time (21:55 GMT), the Pentagon released a second statement saying Mr Austin has "transferred the functions and duties of the office" to Deputy Defence Secretary Kathleen Hicks.
Later on Sunday, the hospital provided an update, saying that, after testing, the secretary had been admitted into the critical care unit "for supportive care and close monitoring".
The bladder issue is not expected to change Mr Austin's "anticipated full recovery", his doctors said, following a prostate cancer diagnosis in December. "His cancer prognosis remains excellent."
Mr Austin was scheduled to travel to Brussels this week for a Wednesday meeting on the ongoing war in Ukraine. It is not yet clear if his hospital stay will impede those plans.
Late last year, Mr Austin had surgery as part of his cancer treatment.
He was readmitted to hospital on New Year's Day with severe pain in his leg, hip and abdomen because of complications related to the procedure. An evaluation found a urinary tract infection and he remained in hospital for more than two weeks.
Senior defence officials and the Biden administration did not know that Mr Austin was seriously ill until three days after his re-admission in January.
During a press conference last week, Mr Austin said he had never directed anyone in his staff to hide his time in hospital from the White House or the public, but did not answer questions about whether staffers had acted outside of his orders.
Still, the defence secretary said he was "deeply sorry" for failing to provide appropriate notice and said he had apologised personally to President Joe Biden.
"I did not handle this right," he said. "I should have told the president about my cancer diagnosis."
The defence secretary is just below the president in the chain of command for the US military, and is regarded as one of the most important members of the cabinet.
The incident incited concerns over both transparency and security and prompted three separate investigations into Mr Austin's handling of his illness and hospital stays. Some prominent Republicans called for Mr Austin to be removed from his post.
President Biden stood by Mr Austin, saying he remained confident in his secretary's leadership, but added that it was "sub-optimal" that he had not disclosed the seriousness of his illness.
He is scheduled to testify before Congress at the end of this month about his failure to notify government leaders.
Mr Austin, a retired four-star general, became the first African-American defence secretary in 2020.
