Western nations warned their citizens on Thursday to immediately leave the surrounds of Kabul airport over a terrorist threat, as thousands of people try to reach a dwindling number of evacuation flights.

Nearly 90,000 Afghans and foreigners have fled Afghanistan via the US-led airlift since the Taliban took control of the country on August 15.

Huge crowds are gathered in and around the airport, becoming increasingly desperate as some foreign nations cease flights ahead of Tuesday's deadline by US President Joe Biden to end the evacuations and withdraw the US troops overseeing it.

One reason for the hard deadline cited by Biden and his aides this week was an “acute” terrorist threat from the regional chapter of the Islamic State (IS) group.

The US government and its allies raised the alarm further on Thursday with a series of coordinated and specific advisories for their citizens to avoid the airport.

“Those at the Abbey Gate, East Gate, or North Gate now should leave immediately,” said the US State Department, citing unspecified “security threats”.

Australia's department of foreign affairs said there was an “ongoing and very high threat of terrorist attack”. “Do not travel to Kabul Hamid Karzai International Airport. If you're in the area of the airport, move to a safe location and await further advice.”

London issued a similar warning, adding “if you can leave Afghanistan safely by other means, you should do so immediately”.

Islamic State threat

IS' Afghanistan-Pakistan chapter has been responsible for some of the deadliest attacks in the two countries in recent years.

It has massacred civilians in both countries, at mosques, shrines, public squares and even hospitals.

The group has especially targeted Muslims from sects it considers heretical, including Shias. But while IS and the Taliban are both hardline Sunni groups, they are rivals and oppose each other.

The Taliban have promised a softer brand of rule from their first stint, which ended in 2001 when the United States invaded because they gave sanctuary to Al Qaeda.

But many Afghans fear a repeat of the Taliban's brutal interpretation of Sharia law, as well as violent retribution for working with foreign militaries, Western missions or the previous US-backed government.

There are particular concerns for women, who were largely banned from education and employment and could only leave the house with a male chaperone during the group's 1996-2001 rule.

“They have not only saved our lives, but they have also saved our dreams,” one member of a girls robotics team said of the Mexican government after fleeing Afghanistan and landing in Mexico City.

“Under this [Taliban] regime, we women will face difficulties [...] that is why we are grateful to be here,” she told reporters.

The crowds at the airport have led to chaos throughout the airlift operations, with thousands of US troops trying to maintain a secure perimeter for evacuation flights.

Some of the Afghans massed outside the airport have foreign passports, visas or eligibility to travel, but most do not.

At least eight people have died in the chaos.

Despite the harrowing scenes, the Taliban have ruled out any extension to next Tuesday's deadline to pull out foreign troops, describing it as “a red line”.

“They have planes, they have the airport, they should get their citizens and contractors out of here,” said Taliban spokesman Zabihullah Mujahid.

Turkey, which had more than 500 non-combat troops stationed in Afghanistan, said on Wednesday it had started pulling out its forces.

The withdrawal signalled an apparent abandonment of the plans it had been negotiating to help secure Kabul's strategic airport after the American departure.

Belgium said on Wednesday it was ending evacuations after its military planes airlifted around 1,100 people — including Europeans and Afghans — in recent days.

France has said it will end its flights on Thursday.

The Pentagon, which is managing all Kabul airport operations, has said it has to wind down evacuations several days before Aug 31.

This is to allow time for the US military to remove its own 6,000-plus troops, plus hundreds of US officials and Afghan security forces, as well as equipment.