In a statement, the Pentagon said Austin was transported by his security detail to Walter Reed National Military Medical Center around 2.20 pm. 

While Austin initially was set to retain the "functions and duties of his office", at about 5 pm on Sunday he transferred those authorities to Deputy Secretary of Defense Kathleen Hicks. As of Sunday evening, he remained hospitalised, said Pentagon Press Secretary Maj Gen Pat Ryder. 

The chairman of the Joint Chiefs of Staff was also notified, as well as the White House and Congress.

Ryder said Austin had travelled to the hospital with the unclassified and classified communications systems needed to perform his job.

Austin was scheduled to depart on Tuesday for Brussels to hold a meeting of the Ukraine contact group, which he established in 2022 to coordinate military support for Kyiv after Russia's invasion. After that, Austin was scheduled to attend a regular meeting of NATO defence ministers. 

It was not immediately clear if this hospitalisation would change those plans. 

Austin was diagnosed with prostate cancer in December and underwent a procedure called a prostatectomy to treat it on December 22. 

Over the next week, he developed complications and on January 1, in extreme pain, he was taken to Walter Reed by ambulance where he was admitted to the intensive care unit. Austin remained at Walter Reed until January 15. He then continued to recover and work from home, and he returned to the Pentagon on January 29. 

His doctors have previously said his prognosis against the cancer is "excellent" and that no further treatments will be needed. 

Austin has gone back to Walter Reed for follow-ups since his hospitalisation but this is his first unscheduled trip due to continued complications from his cancer treatments. 

Austin did not tell President Joe Biden, Congress or his deputy defense secretary of his cancer diagnosis or initial hospitalisation for weeks. That secrecy has become the subject of an inspector general investigation and a Pentagon internal review. He has previously said he never instructed his staff to keep his hospitalisation a secret.