US President Joe Biden has tripped and fallen after handing out the last diploma at a graduation ceremony at the US Air Force Academy but got up quickly and walked back to his seat.

The 80-year-old US president fell forward at Thursday’s ceremony, caught himself with his hands, then got up on one knee helped by three people.

He walked back to his seat unassisted.

After Mr 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 Mr Biden was fine.

“There was a sandbag on stage while he was shaking hands,” he said.

Mr Biden later jogged back to his motorcade when the ceremony ended shortly after the accident.

“I got sandbagged,” he joked as he arrived back at the White House that night.

The fall came after a commencement address Mr 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.

Mr Biden, the oldest person to hold the Oval Office, is running for re-election in 2024.

Polls show US voters are concerned about anyone over 75 becoming president.

Mr Biden has previously acknowledged those concerns as fair. But he maintains he is still up for the job, and urged those who doubt him to keep an eye on his rigorous schedule.

“The only thing I can say is they’re going to see a race, and they’re going to judge whether I have or don’t have it,” Mr Biden said in April after announcing his reelection bid.

His leading Republican opponent, Donald Trump, turns 77 this month. There was unusual sympathy for Mr Biden from Mr Trump

“I hope he wasn’t hurt,” said Mr Trump, who was campaigning in Iowa.

“That’s not inspiring.”

Mr Trump even referenced his own apparent stumble on a stage that made headlines in 2020.

“You got to be careful about that because you don’t – you don’t want that. Even if you have to tiptoe down the ramp,” he said.

At the time, Mr Trump said the ramp at the US Military Academy in West Point, New York, was slippery. He brushed off questions about his own health as fake news.

In February, doctors declared Mr Biden healthy and fit for duty after a physical examination, noting the President does not drink alcohol or use tobacco and exercises “at least” five times a week.

Last June, Mr Biden fell as he was dismounting his bicycle, after snaring a foot in a toe clip. He was uninjured.

-with AAP