Former US President Donald Trump launched on Saturday a sustained attack on President Joe Biden's handling of the retreat of American forces from Afghanistan, which he called "the greatest foreign policy humiliation" in the country's history.

Trump, a Republican who has dangled the possibility of running again for president in 2024, has repeatedly blamed Biden, a Democrat, for Afghanistan's fall to the Islamist militant Taliban, even though the US withdrawal that triggered the collapse was negotiated by his own administration.

"Biden’s botched exit from Afghanistan is the most astonishing display of gross incompetence by a nation’s leader, perhaps at any time," Trump said at a boisterous rally packed with his supporters near Cullman, Alabama.

Taliban leaders are trying to hammer out a new government after their forces swept across the country as US-led forces pulled out after two decades, with the Western-backed government and military crumbling.

For his part, Biden has criticised the Afghan military for refusing to fight, denounced the now-ousted Afghan government and declared he inherited a bad withdrawal agreement from Trump.

At the rally, Trump blamed the situation on Biden not having followed the plan his administration came up with and bemoaned US personnel and equipment being left behind as troops withdrew.

"This is not a withdrawal. This was a total surrender," he said.

Trump said the Taliban, with whom he had negotiated, respected him. He suggested the quick takeover of Afghanistan would not have happened if he was still in office.

"We could have gotten out with honour," Trump added. "We should have gotten out with honour. And instead we got out with the exact opposite of honour."