Joe Biden says he stands “squarely” behind his decision to pull US troops out of Afghanistan.
The US president spoke after the Taliban entered Kabul and swept to power in the country – 20 years after they were removed in the US-led invasion.
Mr Biden said the US mission in Afghanistan was never supposed to be nation-building.
“I stand squarely behind my decision,” he said – though he did admit the collapse of the Afghan government was quicker than anticipated.
He said the rapid end of the Afghan government only vindicates his choice to end the conflict.
He blamed the Taliban’s takeover on Afghan political leaders who fled the country and the Afghan army’s unwillingness to fight.
The president has also blamed his predecessor, Donald Trump, for empowering the Taliban and leaving them “in the strongest position militarily since 2001”.
“American troops cannot and should not be fighting the war, and dying in a war that Afghan forces are not willing to fight for themselves,” Mr Biden added.
The US leader has downplayed the prospect of an ascendant Taliban for months while arguing that Americans of all political persuasions have tired of a 20-year war.
As the Taliban regained control, the US, the UK and other countries scrambled to evacuate their citizens and local allies.
Mr Biden warned the Taliban not to interfere with the US evacuation effort, threatening “devastating force, if necessary”.
Thousands of Afghans rushed into Kabul’s main airport, plunging it into chaos.
Some were so desperate to escape the Taliban that they held on to a military jet as it took off, and fell to their deaths, with senior US military officials claiming seven people died in the chaos.
A US official said soldiers had fired in the air to deter people trying to force their way onto the military flight that was set to take US diplomats and embassy staff out of the fallen city.
Another US official later said forces protecting the airport in Kabul had killed two gunmen in separate incidents.
A third official described the crowd trying to get onto the tarmac at the airport as “out of control”.
And other images showed people dragging children over barbed wire fences, climbing on to an airbridge that allows people to walk on to an aircraft from a terminal and on to the undercarriage housing of a military plane taxiing along the runway.
Other footage distributed by Afghan media and on social media was said to show at least one person falling from the undercarriage of a military plane that had taken off.
A Pentagon spokesman said on Monday night that the airfield at Kabul’s airport had reopened after a pause during the day.
He added that it would be wrong to conclude that the military did not view the Taliban overrunning Afghanistan, including Kabul, as a “distinct possibility”.
The Taliban is expected to announce that Afghanistan is now the Islamic Emirate of Afghanistan, under Sharia law.
Taliban officials have declared war in Afghanistan is over and it is in charge of 90% of government buildings, amid allegations of revenge killings, brutal tactics, and some looting and lawlessness.
President Ashraf Ghani fled the country on Sunday as the Islamists began to enter Kabul virtually unopposed – despite ongoing but short-lived resistance elsewhere – saying he wanted to avoid bloodshed.
Afghanistan’s ambassador to the UN said on Monday that the body’s Security Council must “use every means at its disposal to call for an immediate cessation of violence” in his country, and “respect for human right and international humanitarian law”.
He urged all members of the United Nations not to recognise any administration that achieves power by force or government that is not inclusive.
At a meeting of the UN Security Council convened following the Taliban’s apparent victory, official Afghan ambassador Ghulam Isaczai said: “I’m speaking for millions of Afghan girls and women who are about to lose their freedom… thousands of human rights defenders, journalists, academics, civil servants… whose lives are at risk… thousands of internally displaced people who are in desperate need of shelter and protection.
“We are extremely concerned about the Taliban not honouring their commitments… We’ve witnessed time and again how Taliban have broken their promises in the past. We have seen gruesome images of Taliban mass executions, of military personnel and targeted killings of civilians in Kandahar and other big cities.”
The UK’s Foreign Secretary Dominic Raab said on Monday evening that the UK government was “surprised by the scale and the pace” with which the Taliban took control of Afghanistan.
He revealed that 150 British nationals would be arriving back in the UK in the early hours of the morning, with a further 350 Britons and Afghan nationals arriving in the coming days.
Meanwhile, Prime Minister Boris Johnson intends to host a virtual meeting of G7 leaders and has stepped up efforts to evacuate UK nationals and others from Kabul.
Number 10 said he spoke to French President Emmanuel Macron on Monday about how the two countries could work together to recognise any future Afghan government, as well as try to prevent a humanitarian and refugee crisis.
Many other countries which have been involved in the efforts to rebuild the country amid two decades of war with the Taliban have also started pulling out their diplomatic staff and continued attempting to evacuate their civilians and Afghans who had helped them.
Among the countries planning to pull out staff was Russia, which launched an invasion in 1979 and fought mujahideen including members of the Taliban during a 10-year war. It came as the Taliban deployed guards to the Russian embassy.
The first group of evacuated Britons and embassy staff arrived at RAF Brize Norton on Sunday night, the Ministry of Defence confirmed.
There were reported to be 4,000 Britons in Afghanistan and the UK has said it plans to ramp up efforts to evacuate up to 1,500 people from Afghanistan a day.