Austin effectively vanished from the public eye in late December and again in early January after suffering complications from treatment for a prostate cancer on December 22, having initially concealed both the diagnosis and treatment from President Joe Biden and the rest of the government.

This time, the public was alerted around two hours after Austin was sent to the hospital on Sunday afternoon, with Defense Department press secretary Pat Ryder saying in a statement that military, White House and congressional officials had all been notified.

Austin "was transported by his security detail to Walter Reed National Military Medical Center to be seen for symptoms suggesting an emergent bladder issue," Ryder said.

"The Deputy Secretary of Defense and the Chairman of the Joint Chiefs of Staff have been notified. Additionally, White House and Congressional notifications have occurred."

Austin would be retaining "the functions and duties of his office" while hospitalized, Ryder said.

After coming under heavy political fire for keeping the previous hospital stays secret, Austin apologized earlier this month.

"I should have told the president about my cancer diagnosis," he told journalists on February 1.

At the time, he said he was still in recovery, suffering from leg pain and using a golf cart for transportation inside the Pentagon.

The undisclosed absences -- as well as this current hospital stay -- come at a time when the United States faces a spiraling crisis in the Middle East, with American forces in Iraq and Syria facing near-daily attacks from Iran-backed militants in retaliation for Washington's steadfast support of Israel.

The top US defense official is also a key figure in attempts by the Biden administration to maintain support for Ukraine's fight against Russian invasion, as Republican members of Congress refuse to authorize new funding for military aid to Kyiv.