US Defence Secretary Lloyd Austin told Congress on Tuesday that the Afghan army's sudden collapse caught the Pentagon off-guard as he acknowledged miscalculations in America's longest war including corruption and damaged morale in Afghan ranks.

“The fact that the Afghan army simply melted away in many cases without firing a shot took us all by surprise,” Austin told the Senate Armed Services Committee.

“It would be dishonest to claim otherwise.”

Austin was speaking at the start of two days of what is expected to be some of the most contentious hearings in memory over the chaotic end to the war in Afghanistan, which cost the lives of US troops and civilians and left the Taliban back in power.

The Senate and House committees overseeing the US military are holding hearings on Tuesday and Wednesday, respectively, where Republicans are hoping to zero in on what they see as mistakes that President Joe Biden's administration made towards the end of the two-decade-old war.

Read more: Biden faces mounting blame for Afghan debacle

It follows similar questioning two weeks ago that saw US Secretary of State Antony Blinken staunchly defending the administration, even as he faced calls for his resignation.

Austin praised American personnel who helped airlift 124,000 Afghans out of the country, an operation that also cost the lives of 13 US troops and scores of Afghans in a suicide bombing outside the Kabul airport.

“Was it perfect? Of course not,” Austin said, noting the desperate Afghans who were killed trying to climb the side of a US military aircraft or the civilians killed in the last US drone strike of the war.

Senator James Inhofe, the Senate Armed Services Committee's top Republican, squarely blamed the Biden administration for what critics say was a shameful end to a 20-year endeavour.

Inhofe said Biden ignored the recommendations of his military leaders and left many Americans behind after the US withdrawal.

“We all witnessed the horror of the president's own making,” Inhofe said of Afghanistan.

Many of the hardest questions may fall to the two senior US military commanders testifying: Army General Mark Milley, chairman of the Joint Chiefs of Staff, and Marine General Frank McKenzie, head of US Central Command.