Who are ISIS-K, the Islamic State splinter group who claimed responsibility for airport attack?
British forces are prepared to launch air strikes to target so-called Islamic State terrorists in Afghanistan, the head of the RAF indicated as the US-led military presence in the country came to an end.
The foreign secretary has also not ruled out such strikes against ISIS-K, so who are they and why do the two groups of militants disagree?
Who are ISIS-K?
ISIS-K is an affiliate of the so-called Islamic State terrorist group, which once gained large territories in northern Syria and Iraq.
The splinter group, which was established in 2015, has been mostly based in eastern Afghanistan, part of an area known as the Khorasan province - which is referred to in their name ISIS-K.
Back in 2017, the United States dropped what was known as "the mother of all bombs" in the area to try and send a warning and eliminate the threat.
However, its fighters are thought to number up to 2,200 - although this figure could well be rising given the security vacuum left by departing foreign troops from Afghanistan.
What do we know about ISIS-K?
ISIS-K launched 100 attacks on civilians in Afghanistan and Pakistan between 2015 and 2017, according to a US security think-tank, the Centre for Strategic and International Studies (CSIS).
It has also been responsible for around 250 attacks on US, Pakistani and Afghan forces during that same time period, and since then the number is likely to have risen further.
ITV News Global Security Editor Rohit Kachroo explains: "ISIS-K has operated for six years mostly in eastern Afghanistan in an area called the Khorasan province... and has launched hundreds of attacks on civilians."
What is the threat from ISIS-K?
ISIS-K has carried out a number of high-profile attacks in recent years, despite reports that its numbers had fallen thanks to military action by the US, Afghan forces and the Taliban.
The group is thought to have carried out a devastating attack on a maternity hospital in Kabul in May 2020, killing 24 people including newborn babies and mothers.
Terror attacks are usually unexpected - but this recent one was not, Rohit Kachroo explains
It has also claimed responsibility for several other attacks in Kabul, including an assault on the city's university last November and rocket attacks in the same month. ISIS-K also claimed responsibility for an attack on Jalalabad prison last August.
The group is clearly able to carry out deadly, complex attacks in the area and US secretary of state Antony Blinken described the threat of an ISIS-K attack as "a very real possibility".
What is ISIS-K's relationship with the Taliban and terrorist group al Qaida?
ISIS-K is hostile towards the Taliban which it does not believe is radical enough. The two groups have also previously fought over control of territory in Afghanistan.
ITV News Global Security Editor Rohit Kachroo reports the Taliban is the group's sworn enemy.
After the Taliban's takeover of the country last week, the group reportedly executed a senior ISIS-K commander who had been imprisoned in Kabul.
The conflict between the two groups means that ISIS-K is less likely to be bound by the Taliban's agreement with Western forces to allow evacuations to continue from Kabul airport.
Similarly, the relationship between ISIS-K and al Qaida is unlikely to be one of straightforward co-operation despite their similar beliefs, reflecting broader conflict between al Qaida and IS globally.
What has ISIS-K said?
In 2018, one of ISIS-K's commanders told ITV News Global Security Editor Rohit Kachroo the group have been cooperating with the so-called Islamic State in Syria.
'We will get to their homes and attack them', ISIS-K commander tells ITV News
He said: "Our message to the infidels in Afghanistan and all over the world is that we will get to their cities and homes and attack them."
What has the US said about ISIS-K?
The US confirmed it is the fear of ISIS-K which has forced them to keep to their decision of August 31 as the final date for a withdrawal of Western forces from Afghanistan.
US Secretary of State Anthony Blinken said on Wednesday: "We're operating in a hostile environment in a city and country now controlled by the Taliban with the very real possibility of an ISIS-K attack."
"We're taking every precaution but this is very high risk," he adds.