KABUL (AFP) - As desperate Afghans crowd Kabul airport trying to get on any evacuation flights to flee the Taleban, officials have warned of another militant threat: the Islamic State in Iraq and Syria (ISIS) group.

United States President Joe Biden said there is "an acute and growing risk" of an attack at the airport by the group's regional chapter, called Islamic State-Khorasan or ISIS-K.

The US, Britain and Australia have told people to leave the area for safer locations.

When asked directly about the threat, a Taleban spokesman acknowledged a risk of "nuisances" causing trouble in a chaotic situation they blamed entirely on the US-led evacuation.

What is Islamic State-Khorasan?

Months after ISIS declared a caliphate in Iraq and Syria in 2014, breakaway fighters from the Pakistani Taleban joined militants in Afghanistan to form a regional chapter, pledging allegiance to ISIS leader Abu Bakr al-Baghdadi.

The group was formally acknowledged by the central ISIS leadership the next year as it sunk roots in north-eastern Afghanistan, particularly Kunar, Nangarhar and Nuristan provinces.

It also managed to set up sleeper cells in other parts of Pakistan and Afghanistan, including Kabul, according to United Nations monitors.

Latest estimates of its strength vary from several thousand active fighters to as low as 500, according to a UN Security Council report released last month.

"Khorasan" is a historical name for the region, taking in parts of what is today Pakistan, Iran, Afghanistan and Central Asia.

What kind of attacks has it carried out?

ISIS' Afghanistan-Pakistan chapter has been responsible for some of the deadliest attacks of 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 Shiites.

Last year, it was blamed for an attack that shocked the world - gunmen went on a bloody rampage at a maternity ward in a predominantly Shi'ite neighbourhood of Kabul, killing 16 mothers and mothers-to-be.

Beyond bombings and massacres, ISIS-Khorasan has failed to hold any territory in the region, suffering huge losses because of Taleban and US-led military operations.

According to UN and US military assessments, after the phase of heavy defeats ISIS-Khorasan now operates largely through covert cells based in or near cities to carry out high-profile attacks.

What is ISIS-Khorasan's relationship with the Taleban?

While both groups are hardline Sunni Islamist militants, there is no love lost between them.

They have differed on the minutiae of religion and strategy, while claiming to be the true flag-bearers of jihad.

That tussle has led to bloody fighting between the two, with the Taleban emerging largely victorious after 2019 when ISIS-Khorasan failed to secure territory as its parent group did in the Middle East.

In a sign of the enmity between the two jihadist groups, ISIS statements have referred to the Taleban as apostates.

How has ISIS reacted to the Taleban victory in Afghanistan?

Not well.

ISIS had been highly critical of the deal last year between Washington and the Taleban that led to the agreement for withdrawing foreign troops, accusing the latter of abandoning the militant cause.

Following the Taleban's lightning takeover of Afghanistan, a number of militant groups around the world congratulated them - but not ISIS.

One ISIS commentary published after the fall of Kabul accused the Taleban of betraying Islamic militants with the US withdrawal deal and vowed to continue its fight, according to the Site Intelligence Group, which monitors militant communications.

What is the threat at Kabul airport?

US officials say Kabul airport, with thousands of US-led foreign troops surrounded by huge crowds of desperate Afghans, is under high threat from ISIS-Khorasan.

A flurry of near-identical travel warnings from London, Canberra and Washington late on Wednesday urged people gathered in the area to move to safer locations.

They have not provided any specific details about the threat.

"ISIS-K is a sworn enemy of the Taleban, and they have a history of fighting one another," Mr Biden said on Sunday (Aug 22).

"But every day we have troops on the ground, these troops and innocent civilians at the airport face the risk of attack from ISIS-K."

Some military transports taking off from Kabul airport in recent days have been seen launching flares, which are normally used to attract heat-seeking missiles.