WASHINGTON: U.S. Defense 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 we and our partners trained 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 are 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 U.S. troops and civilians and left the Taliban back in power.

The Senate and House committees overseeing the U.S. 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 toward the end of the two-decade-old war.

It follows similar questioning two weeks ago that saw U.S. 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 U.S. 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 killed trying to climb the side of a U.S. military aircraft or the civilians killed in the last U.S. 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 endeavor. Inhofe said Biden ignored the recommendations of his military leaders and left many Americans behind after the U.S. 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 U.S. military commanders testifying: Army General Mark Milley, chairman of the Joint Chiefs of Staff, and Marine General Frank McKenzie, head of U.S. Central Command.