President Joe Biden has said he stands "squarely" behind the US exit from Afghanistan as he faces withering criticism over the Taliban's lightning conquest of the war-torn country.
"How many more American lives is it worth?" asked the Democratic president.
He said that despite the "messy" pull-out, "there was never a good time to withdraw US forces".
On Sunday, the Taliban declared victory after Afghan President Ashraf Ghani fled and his government collapsed.
The militants' return to rule brings an end to almost 20 years of a US-led coalition's presence in the country.
Kabul was the last major city in Afghanistan to fall to a Taliban offensive that began months ago but accelerated in recent days as they gained control of territories, shocking many observers.
Mr Biden's address followed a dramatic day at Kabul's international airport, where hundreds of civilians desperate to flee the country forced their way inside on Monday.
Many thronged the runway, running alongside a moving military transporter aircraft as it prepared for take-off.
Some clung to the side of a plane, and at least two of them are reported to have perished when they fell from the aircraft after it had left the ground.
American troops killed two armed Afghans who were part of the crowd that breached the airport perimeter. Seven people reportedly died in total.
Latest updates from Afghanistan
Why is Kabul being compared to the fall of Saigon?
The US suspended its evacuation from Kabul but it has now resumed.
A remarkable picture from Sunday appears to show 640 Afghans packed onboard a US military cargo plane leaving Kabul for Qatar.
The image, not verified by the BBC, was obtained by the US defence analysis website, Defense One.
Panicked civilians had scrambled up the loading ramp, the website quoted US officials as saying, but the crew decided it was best to take off rather than force the Afghans off.