Islamic State Khorasan, or ISIS-K, is the regional affiliate of the self-proclaimed Islamic State group active in Afghanistan and Pakistan.
The group on Thursday, and US President Joe Biden has vowed revenge.
"I have also ordered my commanders to develop operational plans to strike ISIS-K assets, leadership and facilities. We will respond with force and precision at our time, at the place we choose and the moment of our choosing," .
Australia has formally listed ISIS-K as a terror organisation since 2017.
Here's a closer look at the group and its history.
What is ISIS-K?
Months after IS declared a caliphate in Iraq and Syria in 2014, breakaway fighters from the Pakistani Taliban joined militants in Afghanistan to form a regional chapter, pledging allegiance to IS leader Abu Bakr al-Baghdadi.
The group was formally acknowledged by the central IS leadership the next year as it sunk roots in northeastern 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.
Hospital staff bring in an injured man on a stretcher for treatment after the Kabul airport attacks Source: AFP
What kind of attacks has it carried out?
IS's 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 Shiite neighbourhood of Kabul, killing 16 mothers and mothers-to-be.
Beyond bombings and massacres, ISIS-K has failed to hold any territory in the region, suffering huge losses because of Taliban and US-led military operations.
What is ISIS-K's relationship with the Taliban?
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 Taliban emerging largely victorious after 2019 when ISIS-K failed to secure territory as its parent group did in the Middle East.
In a sign of the enmity between the two jihadist groups, IS statements have referred to the Taliban as apostates.
How has IS reacted to the Taliban victory in Afghanistan?
Not well.
IS had been highly critical of the deal last year between Washington and the Taliban that led to the agreement for withdrawing foreign troops, accusing the latter of abandoning the jihadist cause.
Following the Taliban's lightning takeover of Afghanistan, a number of jihadist groups around the world congratulated them - but not IS.
One IS commentary published after the fall of Kabul accused the Taliban of betraying jihadists with the US withdrawal deal and vowed to continue its fight, according to the SITE Intelligence Group, which monitors militant communications.
Additional reporting by SBS News.
