US Secretary of Defence Lloyd Austin was placed under general anaesthetic on Monday after he was taken to hospital for a bladder problem that flared up during his recovery from prostate cancer.
The Pentagon chief had to cancel a planned trip to Brussels on Tuesday to hold a meeting of the Ukraine contact group, which he established in 2022 to co-ordinate military support for Kyiv after Russia's invasion.
He also will no longer be meeting Nato defence ministers at a separate summit, Pentagon spokesman Maj Gen Pat Ryder told reporters.
Mr Austin was sent to Walter Reed National Military Medical Centre and admitted to the critical care unit late on Sunday, two specialists said.
"Mr Austin underwent non-surgical procedures under general anaesthesia to address his bladder issue," Dr John Maddox and Dr Gregory Chesnut said in a statement.
"We anticipate a successful recovery and will closely monitor him overnight."
A long stay in hospital was not expected and Mr Austin was due to resume his duties on Tuesday, the doctors said.
The Pentagon chief on Sunday transferred his duties to deputy defence secretary Kathleen Hicks.
The chairman of the Joint Chiefs of Staff was notified of Mr Austin's latest health problem, as were the White House and some members of Congress.
He apologised this month for concealing the diagnosis, saying he did so because he felt it was a private matter.
“I should have told the President about my cancer diagnosis,” he said.
Maj Gen Ryder said Mr Austin is expected to make a full recovery from the cancer.
"There is no further treatment associated with his cancer expected, other than the physical therapy that he's been undergoing to deal with some lingering leg pains," he said.