Western nations scrambled to evacuate people from Afghanistan on Wednesday but acknowledged that many Afghans who helped during two decades of war would be left behind to face an uncertain fate under the Taliban when foreign troops leave next week.

The effort was taking place against a chaotic backdrop in the capital, Kabul, and its airport, where a massive airlift of foreign nationals and their families as well as some Afghans is under way and due to last until Tuesday.

Pressure to complete the evacuations has intensified amid concerns about the security situation outside the airport, where the Taliban are in control.

In an alert issued on Wednesday evening, the US Embassy in Kabul advised US citizens to avoid traveling to the airport. It said those already at a number of the airport's gates should leave immediately. It did not give a reason why the alert was issued.

US President Joe Biden has ordered all troops out of the South Asian nation by the end of the month, spurning pleas from European allies for more time to get people who helped NATO countries during the conflict to safety.

In the 10 days since the Taliban swept into Kabul, the United States and its allies have mounted one of the biggest air evacuations ever, bringing out more than 88,000 people, including 19,000 in the past 24 hours. The US military says planes are taking off the equivalent of every 39 minutes.

Biden, implementing a withdrawal negotiated by his predecessor, Donald Trump, said US troops in Afghanistan faced mounting danger if they stayed.

The White House said Biden was briefed on Wednesday on contingency plans for the Afghanistan evacuation as well as the threat from the ISIS-K militant group.

Britain's Foreign Office said the security situation in Afghanistan remained volatile, with a "high threat of a terrorist attack."

US Secretary of State Antony Blinken said at least 4,500 American citizens and their families had been evacuated from Afghanistan since mid-August, and the State Department was reaching out to about 1,500 who remained there.

Blinken told a news conference in Washington there was no deadline for the effort to help people who want to leave, both Americans and others, and that it would continue for "as long as it takes."

The US military said it would shift its focus to evacuating its own troops in the final two days before the August 31 withdrawal deadline.

The Taliban have said foreign troops must be out by the end of the month. They have encouraged Afghans to stay, while saying those with permission to leave will still be allowed to do so once commercial flights resume after the foreign troops go.

Several of Washington's European allies say they no longer have time to rescue all of the many thousands of Afghans who helped their forces and their diplomatic and aid efforts.

British foreign minister Dominic Raab said the deadline for evacuating people was up to the last minute of the month. France said it would push on with evacuations as long as possible but was likely to end them in the coming hours or days. Chancellor Angela Merkel said Germany would try to help Afghans even after the deadline expired.

Seeking flights out

Tens of thousands of Afghans fearing persecution have thronged Kabul's airport seeking flights out since the takeover by the Islamist militants.

On Wednesday, many people milled about outside the airport - where soldiers from the United States, Britain and other nations were trying to maintain order amid the dust and heat.

They carried bags and suitcases stuffed with possessions, and waved documents at soldiers in the hope of gaining entry. One man, standing knee-deep in a flooded ditch, passed a child to a man above.

"I learned from an email from London that the Americans are taking people out. That's why I've come, so I can go abroad," said one man, Aizaz Ullah.

While the focus is now on those trying to flee, Afghans who remain behind face a dire situation marked by a rising threat of food insecurity and disease, aid groups say. The U.N. World Food Programme estimates about 14 million people face the threat of starvation.

There are also reports of serious human rights violations by the Taliban, including executions of civilians and Afghan security forces who had surrendered. The Taliban have said they will investigate any reported atrocities.

The United Nations is leaving some 3,000 Afghan staff at its mission. A U.N. security document reviewed by Reuters described dozens of incidents of threats, the looting of U.N. offices and physical abuse of staff since August 10.

The Taliban's 1996-2001 rule was marked by public executions and the curtailment of political rights and basic freedoms. Women were barred from school or work.

Afghanistan was also a hub for anti-Western militants, and Washington, London and others fear it might become so again. The Taliban have said they will respect human rights including those of women, and not allow terrorists to operate from the country.

'What will they do to us?'

"Every woman I know has the same fear as I do. What will now happen to our children if we are punished for our work? What will happen to our families? What will they do to us as women?" an Afghan woman who has worked for the United Nations for several years told Reuters on condition of anonymity.

The Taliban are seeking to persuade people at the airport to go home, saying they have nothing to fear.

"Foreign troops should withdraw by the deadline. It will pave the way for resumption of civilian flights," Taliban spokesman Suhail Shaheen said on Twitter. "People with legal documents can travel through commercial flights after August 31."

The Taliban have asked NATO member Turkey to help keep the airport open after foreign troops leave, but have also said no Turkish troops will be permitted to stay. Turkey, which began evacuating its military on Wednesday, said technical experts might remain to help operate the airport.

The US-backed Afghan government collapsed swiftly after Biden withdrew the troops, two decades after they ousted the Taliban in the weeks following the September 11, 2001, attacks on the United States, planned from Afghan territory by al Qaeda.

The Taliban are switching focus to how to run a country in crisis. Key posts are expected to be filled by loyal veterans of their insurgency.

According to Afghanistan's Pajhwok news agency, the future finance minister will be Gul Agha, who is on international sanctions lists for acting as the insurgents' financial chief. Al Jazeera news channel reported the new defence minister would be Mullah Abdul Qayyum Zakir, a former detainee at the US prison in Guantanamo Bay, Cuba.