Austin, who was admitted to Walter Reed on January 1 and spent two weeks in the military hospital, spoke via videolink from his residence at the opening of a meeting on aid for Ukraine's fight against invading Russian forces.

"The security of the entire international community is on the line in Ukraine's fight. I am more determined than ever to work with our allies and partners to support Ukraine and to get the job done," Austin said in front of a Defense Department seal and small American and Ukrainian flags.

He highlighted a $250 million military assistance package that Washington announced last month, but did not detail any new American aid, with funding having dried up and Republicans refusing to authorize more until President Joe Biden bows to their demands for measures to curb migration over the Mexican border.

Austin, a 70-year-old career soldier underwent minor surgery to treat prostate cancer on December 22, returning home the following day, but was readmitted less than two weeks later for complications including nausea and severe pain.

The White House was not informed about Austin's hospitalization until January 4, while Congress was not told until the following day, and Biden did not learn of the cancer diagnosis until January 9.

In response, White House chief of staff Jeff Zients ordered an urgent review of the rules for when senior US officials are incapacitated.

The Pentagon's independent inspector general also announced a review "to examine the roles, processes, procedures, responsibilities, and actions related to the secretary of defense's hospitalization" in both December and January.

The secretary's undisclosed hospitalization left a key national security official unaccounted for at a time when Washington's forces are frequently under fire in Iraq and Syria, and Yemen's Huthi rebels are attacking international shipping in the Red Sea.

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