Lloyd Austin (centre) speaks during a hearing in Washington on Tuesday. AP

US Defence 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. Meanwhile, top US generals said on Tuesday that they advised Biden to keep American troops in Afghanistan and expressed concern that the Taliban has not severed ties with Al Qaeda.

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Top US military leaders to face Congress on Afghan pullout decision

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General Mark Milley, the chairman of the Joint Chiefs of Staff, and General Kenneth McKenzie, commander of US Central Command, said they had personally recommended that some 2,500 troops remain on the ground in Afghanistan.

Chairman of the Joint Chiefs of Staff Gen. Mark Milley listens to a Senator's question during a hearing on Tuesday. AP

It was their first public congressional testimony since the Taliban won the war in August.

"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."

Biden, in April, ordered a complete pullout of US forces from the country by August 31, following through on an agreement reached with the Taliban by former president Donald Trump.

General Milley noted military warnings since late 2020 that an accelerated, unconditional withdrawal could precipitate the collapse of the Afghan military and government.

Milley, McKenzie and Defence Secretary Lloyd Austin were grilled for nearly six hours by members of the Senate Armed Services Committee about the US withdrawal from Afghanistan and the chaotic evacuation from Kabul airport.

Republican Senator Tom Cotton asked Milley why, if everything he said was true, he didn't resign his position.

Milley pushed back strongly, saying a US president does not have to agree with the advice of his generals. "This country doesn't want generals figuring out what orders we are going to accept and do or not. That's not our job," Milley said.

General Milley noted military warnings since late 2020 that an accelerated, unconditional withdrawal could precipitate the collapse of the Afghan military and government.

Austin, Milley and senators — many of whom oversaw the war effort for years — seemed full of questions about what went wrong, citing failures to appreciate the impact of corruption and damaged morale in the ranks.