US President Joe Biden has been slammed online after wandering off stage during a campaign event because he "couldn't resist" a baby he spotted in the audience.

At a campaign event in Phoenix, Arizona, on Tuesday, President Joe Biden unexpectedly left the stage during campaign manager Julie Chávez Rodríguez's speech.

Captivated by a baby in the audience, Biden took a seat beside the child and its mother.

He later returned to the stage to speak before admitting he "couldn't resist that little baby".

"Well, folks, I have to tell you straight up I like you all, but I couldn’t resist that little baby," he said.