• He had been due to lead a meeting of the Ukraine Defence Contact Group (UDCG) in Brussels, Belgium.
• He was also set to attend a meeting of the Nato Defense Ministerial chaired by Nato chief Jens Stoltenberg on Thursday.
US Defence Secretary Lloyd Austin has cancelled an overseas trip to Nato headquarters after being admitted to hospital for the third time in as many months.
Mr Austin, 70, is in a critical care unit in a Washington DC area hospital. The Pentagon has said he is facing an "emergent bladder issue".
The cabinet member's duties have been transferred to his deputy.
He has faced scrutiny over the secrecy of his previous hospital admissions.
Mr Austin failed to disclose his prostate cancer diagnosis in December or his return to hospital in January, either publicly or to key figures in the US chain of command.
On Wednesday, he had been due to lead a meeting of the Ukraine Defence Contact Group (UDCG) in Brussels, Belgium. It would have been his first overseas trip since his initial hospital admission.
He was also set to attend a meeting of the Nato Defense Ministerial chaired by Nato chief Jens Stoltenberg on Thursday.
Mr Austin attended the last meeting of the UCDG virtually from his home on 23 January.
On Sunday afternoon, the Pentagon released a statement saying that Mr Austin had been taken to Walter Reed National Military Medical Center for treatment, adding that the White House and senior defence officials had been notified.
Later that afternoon, 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 the secretary had been admitted into the critical care unit "for supportive care and close monitoring".
The bladder issue was not expected to change Mr Austin's "anticipated full recovery" from cancer, his doctors said.