Former US president Donald Trump used the 20th anniversary of the 9/11 attacks to slam the "inept administration" of Joe Biden for its "incompetence" over the Afghan withdrawal in a video message on Saturday.

"This is a very sad day," Trump said in the message, adding that September 11 "represents great sorrow for our country."

"It is also a sad time for the way our war on those that did such harm to our country ended last week," he continued.

Trump was referring to the end of the US war in Afghanistan, launched in the wake of Al-Qaeda's attacks on the World Trade Center in New York and the Pentagon in Washington DC.

Al-Qaeda had been sheltering in Taliban-held Afghanistan, and the US invasion toppled the Islamist regime in a bid to find the group's leaders.

But the Taliban soon launched an insurgency and, after two decades of war in which Afghan civilians paid an outsized price, stormed back to power last month as the US withdrew all its troops.

"The leader of our country was made to look like a fool and that can never be allowed to happen," he said.

He blamed "bad planning, incredible weakness and leaders who truly didn't understand what was happening."

Trump also lamented the deaths of 13 US troops in a bomb blast in Kabul last month during the frenzied evacuation from Afghanistan, and the billions of dollars in US military equipment that was left behind and seized by the Taliban "without a shot being fired."

"Joe Biden and his inept administration surrendered in defeat," Trump continued. 

"We will struggle to recover from the embarrassment this incompetence has caused."