President Joe Biden tripped and fell after handing out the last diploma at a graduation ceremony at the U.S. Air Force Academy on Thursday, but got up quickly and walked back to his seat.

The 80-year-old U.S. president fell forward, caught himself with his hands, then got up on one knee helped by three people. He walked back to his seat unassisted.

After Biden was helped up, he pointed behind him, seeming to indicate what he tripped over. He mingled with other officials afterward, smiling and giving a "thumbs up" sign.

White House communications director Ben LaBolt said on Twitter that Biden was fine. "There was a sandbag on stage while he was shaking hands," he explained.

The fall came after a commencement address

Biden delivered to a flag-waving audience where he warned graduates they will enter service in an increasingly unstable world, citing challenges from Russia and China.

Biden, the oldest person to hold the Oval Office, is running for re-election in 2024. Polls show Americans are concerned about anyone over 75 becoming president. His leading Republican opponent, Donald Trump, turns 77 this month.

Doctors declared Biden healthy and fit for duty after a physical examination in February, noting the President does not drink alcohol or use tobacco and exercises "at least" five times a week.

Biden fell as he was dismounting his bicycle last June, after snaring a foot in a toe clip, but was uninjured.