Deputy Secretary of Defense Kathleen Hicks "assumed the functions and duties" just before 5:00 pm (2200 GMT), Pentagon spokesman Pat Ryder said in a statement, adding that Austin was still receiving treatment and that the White House and Congress had been informed.

The announcement came weeks after it emerged that Austin, 70, had kept previous hospital stays secret and had not immediately informed US President Joe Biden of his cancer diagnosis, sparking criticism as the United States faces crises in the Middle East and Ukraine.

Austin effectively vanished from the public eye for treatment for prostate cancer in December and again in January after suffering complications from the procedure.

This time, the public was alerted around two hours after he was sent to the hospital on Sunday afternoon.

Austin "was transported by his security detail to Walter Reed National Military Medical Center," Ryder said in an earlier statement.

"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."

Initially Ryder noted that the defense chief brought along classified communications systems and would be retaining "the functions and duties of his office."

The announcement that Hicks would instead be taking over came just a few hours later, however. Ryder said the Pentagon would give more updates as soon as possible.

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.

- Low profile -

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 has gained a reputation as a largely apolitical public servant who is most comfortable with US troops.

A commanding presence at well over six feet tall, he 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."