US Defense Secretary Lloyd Austin told Congress on Tuesday that the Afghan army's sudden collapse caught the Pentagon "by surprise," as military leaders confronted a contentious Senate hearing about how and why America lost its longest war. 

Republican lawmakers accused President Joe Biden of lying about recommendations from his military that some troops should be kept in Afghanistan. Even Biden's Democrats expressed frustration with a chaotic withdrawal that left US troops dead and American citizens behind. 

Biden's approval ratings have been badly damaged by last month's spectacular collapse of the two-decade war effort, with painful images of Afghans clinging desperately to a US military plane as they tried to escape from Afghanistan. 

General Mark Milley, chairman of the Joint Chiefs of Staff, and General Frank McKenzie of US Central Command also acknowledged being caught off-guard by the speed of their takeover and collapse of the US-backed government in Kabul. 

"The fact that the Afghan army we and our partners trained simply melted away – in many cases without firing a shot – took us all by surprise," Austin, a former four-star general who served in Afghanistan, told the Senate Armed Services Committee. 

"It would be dishonest to claim otherwise." 

McKenzie and Milley testified that they had believed it would have been best to keep a minimum of 2,500 troops in the country. In an August interview, Biden denied his commanders had recommended that, saying: "No. No one said that to me that I can recall." 

"The loss of our service members, and abandonment of Americans and Afghan allies last month was an unforced, disgraceful humiliation that didn't have to happen," Republican Senator Joni Ernst said. 

Senator James Inhofe, the panel's top Republican, described it as a "horror of the president's own making."