US President Joe Biden has threatened a "swift and forceful" response if the Taliban attacks his country's personnel or disrupts the military operations under way at Kabul airport.
Biden called the scenes of desperation unfolding in Kabul "gut-wrenching" on Monday, in his first comments since the Taliban took full control of Afghanistan.
But he insisted he still stands "squarely behind" his decision to withdraw US troops from Afghanistan and argued it would have made no difference if they had stayed longer.
"After 20 years, I've learned the hard way that there was never a good time to withdraw," Biden said.
Biden admitted that Washington underestimated the pace of the Taliban's advance.
But he also pointed to the Afghan military giving up key cities without a fight in recent days, and argued that while the US had trained and equipped Afghan security forces, they could not give them the will to fight for their future.
He said the US should be "narrowly focused on counter-terrorism, not counter-insurgency or nation-building."
The US president also allocated up to 500 million dollars to meet "unexpected urgent refugee and migration needs of refugees, victims of conflict, and other persons at risk as a result of the situation in Afghanistan," in a memorandum addressed to US Secretary of State Antony Blinken and released by the White House.
Meanwhile US forces were struggling to restore order at Kabul's airport on Monday.
In the wake of the Taliban takeover, US troops tried to facilitate the evacuation of Western diplomats while holding back a crush of Afghans also trying to flee the country.
The disarray at the airport meant forces there would spend much of the rest of Monday restoring order before departures can resume on Tuesday, said US deputy national security adviser Jon Finer, speaking to MSNBC.
The US military has so far deployed around 2,500 soldiers at Kabul airport and will increase to 3,000 in the next 24 hours, Pentagon Press Secretary John Kirby said.
The goal is to have about 6,000 troops on hand to assist with evacuation.
But that job will be complicated by the hundreds of Afghans who have gathered at the airport and are rushing any departing flight, desperate to escape the country before the Taliban assert full control.
The Taliban ruled the bulk of Afghanistan for about five years at the end of the 20th century. During that time, they enforced a strictly Islamist society, which meant most women were banned from public life and men could be punished for not displaying enough piety. Perceived vices, such as alcohol and Western music, were banned.
Although the Taliban have said they will not be as strict this time, few believe them and are not willing to take their chances under a renewed Taliban regime.
While some Western governments have vowed to try to bring home interpreters and other Afghan locals who have helped them, it is not clear if there is enough time or space to get them all out. Many people at the airport have no papers, so it's impossible to tell most people's status.
Belgium, the Czech Republic, Denmark, Italy, the Netherlands, Norway, the Philippines, Sweden, France and Germany were among the nations trying to get people out on Monday.
There have been reports of deaths and shooting as US troops seek to maintain order.
The Pentagon said two armed Afghans - not from the Taliban - were killed at the airport by US troops in self-defence.
According to unsubstantiated reports, some people hid in aircraft wheel wells and fell to their deaths at takeoff. One Afghan soldier reported forcing his way onto a plane with other Afghans and ending up in Doha, with no identification or plan.
End of '20-year exercise'
Kabul's collapse on Sunday signalled the end of a 20-year-long nation-building exercise led by the US with support of NATO allies.
The US kicked the Taliban out of Kabul in 2001, because the Taliban had provided shelter to the al-Qaeda operatives who planned and took part in hijacking the airplanes that were used to destroy the World Trade Center in New York and the Pentagon outside Washington on September 11 of that year.
Former US president Donald Trump's administration agreed to withdraw in exchange for the Taliban making peace with the Afghan government, and his successor Biden stuck to the plan, aside from delaying it a few months.
However, as foreign troops began leaving, Taliban troops began their push. Afghan forces gave up multiple key cities without a fight and Western forces are now leaving as fast as they can.
On Monday, residents reported that Taliban representatives were taking over police stations and offices across Kabul, often traveling in official vehicles. There were also new checkpoints in the city, residents reported.
Representatives of the collapsed Afghan government and the Taliban are holding talks, said a spokesperson for former Afghan president Hamid Karzai.
No Taliban comment was available.