US President Joe Biden has tripped and fallen while handing out diplomas at a graduation ceremony for the US Air Force Academy in Colorado.

Mr Biden, who is the nation's oldest serving president at 80, was seen being helped up by Air Force officials and walking back to his seat unassisted.

The president had been standing for about an hour and a half to shake hands with each of the 921 graduating cadets.

The White House communications director said "he's fine".

"There was a sandbag on stage while he was shaking hands," Ben LaBolt wrote on Twitter after Thursday's fall.

A White House press pool report also earlier said Mr Biden had tripped on a black sandbag while moving on stage.

Footage of the incident shows Mr Biden appearing to point at the sandbag as he gets back on his feet.

He was seen jogging back to his motorcade, apparently uninjured, when the ceremony ended shortly after the accident.

The pool reporter added that the president did not take questions when he returned to his plane. White House press secretary Karine Jean-Pierre said Mr Biden was feeling "totally fine" and had boarded the plane flashing "a big smile".

Critics have said Mr Biden is too old to run for a second term as president.

Recent polls suggest a majority of US voters are concerned about his advanced age. He would be 82 at the start of a second term if he wins.

This fall, in addition to previous stumbles from his bicycle and on the way up the Air Force One stairs, could add to those concerns.

Former President Donald Trump, the Republican frontrunner to face Mr Biden in the 2024 White House election, reacted to the incident from a campaign event in Iowa, saying "the whole thing is crazy".

"I hope he wasn't hurt," said Mr Trump, 76, who has often poked fun at Mr Biden's age. "That's not inspiring."

"You got to be careful about that because you don't - you don't want that. Even if you have to tip toe down the ramp," added Mr Trump, apparently referring to his own careful walk off a stage that made headlines in 2020.

He said at the time that the ramp at the US Military Academy in West Point, New York, was slippery, and brushed aside the ensuing media questions about his own health as fake news.

Mr Biden's last physical examination was conducted this February.

White House physician Dr Kevin O'Connor wrote at the time: "The President remains fit for duty, and fully executes all of his responsibilities without any exemptions or accommodations."