SHANKSVILLE, PENNSYLVANIA (AFP) - United States President Joe Biden, speaking unexpectedly during a visit to the Pennsylvania site of one of the 9/11 plane crashes, again defended the widely criticised withdrawal from Afghanistan, saying the US could not "invade" every country where Al-Qaeda is present.

"Could Al-Qaeda come back (in Afghanistan)?" he asked in an exchange with reporters outside a Shanksville fire station. "Yeah. But guess what, it's already back in other places.

"What's the strategy? Every place where Al-Qaeda is, we're going to invade and have troops stay in? C'mon."

Mr Biden said it had always been a mistake to think Afghanistan could be meaningfully united.

Mr Biden said American forces had achieved their central mission when a special forces team killed Al-Qaeda founder Osama bin Laden on May 2, 2011, in a compound in Pakistan.

The US intervention in Afghanistan began after the Sept 11, 2001, terror attacks, eventually drawing the US - joined by key allies - into its longest war.

Mr Biden had begun his day on Saturday in Manhattan, attending a televised ceremony marking the Sept 11 attacks there.

He had not been scheduled to make public remarks. But asked by a reporter about the chaotic withdrawal from Afghanistan and a subsequent drop in his poll numbers, he shrugged it off.

"I'm a big boy," Mr Biden said. "I've been doing this a long time."

But he also alluded clearly to one source of that criticism, former president Donald Trump.

Referring to "the stuff that's coming out of Florida", he mentioned a recent statement that if General Robert E. Lee - who led the troops of the pro-slavery Confederacy during the Civil War - "had been in Afghanistan, we would have won".

The assertion about Lee came in a statement from Mr Trump, who now lives in Florida.