Austin effectively vanished from the public eye for treatment for prostate cancer in December and again in January after suffering complications from the procedure, 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 the 70-year-old 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," 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."

It was not immediately clear whether Austin had been admitted yet or how long he would stay, though Ryder noted that the defense chief brought along classified communications systems and would be retaining "the functions and duties of his office."

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.

Various Republican lawmakers previously called for Austin to be sacked, but Biden, while lamenting the Pentagon chief's lapse in judgment, has said he remains confident in his defense secretary.

Austin -- a commanding presence at well over six feet tall -- is an intensely private person who eschews the spotlight, which he said played into his decision to keep the cancer diagnosis secret.

But he admitted that "taking this kind of job means losing some of the privacy that most of us expect. The American people have a right to know if their leaders are facing health challenges that might affect their ability to perform their duties, even temporarily."