US President Joe Biden has been caught on a live microphone calling a Fox News journalist a "stupid son of a b****" on the sidelines of a White House photo op.

As journalists were leaving the room after the event, a reporter from Fox News, a favourite channel of conservatives in the US, asked whether inflation is a political liability.

The Democratic leader, possibly unaware that his microphone was still on, began by answering in a deadpan fashion: "It's a great asset. More inflation."

He then muttered: "What a stupid son of a b****," before glancing briefly down.

A pool reporter who was in the room at the time admitted to not being able to hear what Mr Biden actually said over the noise.

But he added that he would "direct your attention to video of the event if you are curious how the president really feels about being asked about inflation from Fox's Peter Doocy."

Mr Doocy shrugged the insult off in a later interview on Fox.

"Yeah nobody has fact-checked him yet and said it's not true," he said, nonchalantly.

Mr Doocy later said that Mr Biden called him within the hour and said: "It's nothing personal, pal."

When Mr Biden has gaffed before, the White House has rushed to explain or roll-back his comments.

But this time, the White House appeared to have no qualms about owning it, putting out a transcript of the event that included the comment - thereby ensuring it passes into the official historical record.