US President Joe Biden quipped that he got "sandbagged" after he tripped and fell – but was uninjured – while on stage at a US Air Force Academy graduation.

Biden had been greeting graduates in Colorado Springs, Colorado, at the front of the stage with salutes and handshakes, and turned to jog back toward his seat when he fell. 

He was helped up by an air force officer as well as two members of his US Secret Service detail.

Onlookers, including some members of the official delegation onstage, watched in concern before Biden, who at age 80 is the oldest president in US history, returned to his seat to view the end of the ceremony.

"I got sandbagged," the president told reporters with a smile when he arrived back at the White House before pretending to jog into the residence.

Two small black sandbags had been onstage supporting the teleprompter used by Biden and other speakers at the graduation.

"He's fine," White House communications director Ben LaBolt tweeted after the incident. "There was a sandbag on stage while he was shaking hands."

President Joe Biden falls on stage during the 2023 United States Air Force Academy Graduation Ceremony in Colorado Springs, Colorado. – AP

Biden has been dogged by questions about his age and his fitness to serve, and his missteps have become fodder for political rivals as he campaigns for a second term in 2024. 

He has stumbled before going up the stairs and onto Air Force One and he once got caught up in his bike pedals while stopping to talk to reporters near his home.

Biden's personal doctor said after the president's most recent physical exam in February that Biden "remains a healthy, vigorous 80-year-old male, who is fit to successfully execute the duties of the Presidency". 

Biden, the oldest person to hold the Oval Office, is running for re-election in 2024. His leading Republican opponent Donald Trump turns 77 this month.

Asked about Biden's fall during an Iowa rally, Trump said: "He actually fell down? Well I hope he wasn’t hurt."

Trump added: "You gotta be careful about that", even if you have to "tip-toe down a ramp."

In June of 2020, Trump's slow, careful descent of a ramp at the US Military Academy at West Point had some observers questioning his agility. "The last thing I was going to do is ‘fall’," Trump tweeted afterward.

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.

A Cadet receives her diploma as she salutes President Joe Biden during the United States Air Force Academy graduation ceremony at Air force Academy, Colorado. – AP