Two top US generals have said they recommended keeping a force of 2,500 troops in Afghanistan, ahead of the full US withdrawal in August.
Testimony by Gen Mark Milley and Gen Frank McKenzie to Congress seemed to contradict President Joe Biden, who said he did not recall any such advice.
The Taliban took power in August, after rapidly advancing through the country.
Gen Milley said the US had been taken by surprise by the speed of the Afghan government's collapse.
The two US generals were questioned by the Senate armed services committee along with Defence Secretary Lloyd Austin on Tuesday.
The hearing comes weeks after a chaotic withdrawal at Kabul airport as foreign powers sought to get their citizens home and thousands of desperate Afghans begged for rescue.
A suicide attack killed 182 people during the withdrawal operation. Thirteen US service personnel and at least 169 Afghans were killed by the airport gate on 26 August.
Gen McKenzie, who as head of US Central Command oversaw the withdrawal from Afghanistan, said under questioning from Republican senators that he recommended keeping a small force of 2,500 troops in Afghanistan.
This appears to contradict President Joe Biden's assertion to an ABC journalist on 19 August that he did not recall anyone giving him such advice.
Gen Milley said that he agreed with the recommendation, but when asked by Alaska Republican Dan Sullivan whether Biden's comments were "a false statement", he refused to give a direct answer.
