"There was nothing that I, or anyone else, saw that indicated a collapse of this army and this government in 11 days," US Joint Chiefs Chairman General Mark Milley said.

"The Afghan security forces had the capacity, and by that I mean they had the training, the size, the capability, to defend their country. This comes down to an issue of will and leadership," he added.

The US military and the administration of President Joe Biden are under political attack domestically over the Taliban's defeat of the Afghan forces with little fight and the collapse of president Ashraf Ghani's US-backed government last weekend.

The speed appeared to catch the US government off guard and it launched a rapid evacuation operation for US citizens and Afghans granted special visas for their work for US forces.

Since Saturday, around 5,000 US troops have flown in to Kabul's Hamid Karzai International Airport to manage evacuations of thousands.

Critics have faulted the State Department, US intelligence and the Pentagon for not anticipating the debacle and preparing earlier for the evacuation, which involves more than 10,000 US citizens.

Douglas London, the CIA's former counterterrorism chief for South Asia and then an advisor to Biden's presidential campaign, said US intelligence had predicted the Taliban would defeat Afghan forces and that it was possible the government would capitulate within days.

Those projections were "highlighted to Trump officials and future Biden officials alike," in the last year, London wrote Wednesday on the Just Security website.