Watch: Suspected suicide bombers strike outside Kabul airport
The Afghan affiliate of Islamic State, known as Islamic State Khorasan (ISIS-K), is believed to be responsible for the attacks on Thursday.
It followed warnings over the past few days that ISIS-K would launch a strike in the final phase of the Afghanistan evacuation effort.
Reuters reported the US government "strongly believes" ISIS-K was behind the attack, with one official saying the twin suicide bombings have “all the hallmarks” of the group.
ISIS-K is not to be confused as being allies of the Taliban, which seized Afghanistan earlier this month. The two groups are sworn enemies.
Here is what you need to know about ISIS-K.
According to the Center for Strategic and International Studies (CSIS), ISIS-K has been active since 2015.
It is said to have been founded by former Taliban commanders and militants from Pakistan.
The “K” part of its name is after the historical Khorasan region of central Asia. However, ISIS-K now primarily operates in the north and east of Afghanistan – in proximity to Kabul.
Its leader, Aslam Farooqi, is alleged to have directed attacks against civilians in the country, and was arrested in April last year. The Human Rights Watch organisation called for him to be tried for war crimes.
However, amid the chaos in Afghanistan following the Taliban's takeover, his current whereabouts are unknown.
In the three years up to 2018, the CSIS said ISIS-K had been responsible for 100 attacks against civilians in Afghanistan and Pakistan, with 250 clashes with US, Afghan and Pakistani forces.
Deadly attacks have become regular occurrences.
The group is thought to have carried out a devastating attack on a maternity hospital in Kabul in May last year, killing 24 people including newborn babies and mothers.
It has also claimed responsibility for several other attacks in Kabul, including an assault on the city’s university in November last year and a car bomb which killed 80 mostly female students outside a school in May.
Three female media workers were also shot dead in the eastern Afghan city of Jalalabad in March, with ISIS-K claiming responsibility.
A recent UN Security Council report suggested there were up to 2,200 ISIS-K fighters.
ISIS-K is even more hardline than the Taliban, which seized Afghanistan earlier this month, with a more extreme version of Islam.
The two groups are sworn enemies and have previously fought over control of territory in Afghanistan.
After the Taliban’s takeover, the group reportedly executed a senior ISIS-K commander who had been imprisoned in Kabul.
The conflict between the two groups means ISIS-K won't be bound by the Taliban’s agreement with Western forces to allow evacuations to continue from Kabul airport.
The Taliban has been guarding the perimeter outside Kabul airport, having announced on Tuesday it will no longer allow Afghan nationals to leave the country.
Before the attacks on Thursday, a Taliban official, speaking on condition of anonymity, had told Reuters of the ISIS-K threat: “Our guards are also risking their lives at Kabul airport, they face a threat too from the Islamic State group.”
Kabul airport threat
Reports of the ISIS-K threat to Kabul airport began to emerge on Friday last week, when White House national security adviser Jake Sullivan said: "One of the contingencies we are very focused on, laser focused on, is the potential for a terrorist attack by a group like ISIS-K."
On Tuesday, after UK prime minister Boris Johnson tried and failed to convince US president Joe Biden to keep US troops in Kabul past the 31 August deadline, Biden told G7 leaders the threat of ISIS-K was one of the reasons he was withdrawing.
Biden's press secretary Jen Psaki, giving an overview of the president's contribution to the virtual summit, said: “He also made clear that with each day of operations on the ground, we have added risk to our troops with increasing threats from ISIS-K."
The president was also briefed about the threat of ISIS-K on Wednesday.
However, the former head of British forces in Afghanistan had argued earlier on Thursday that ISIS-K is not necessarily “the main threat”.
Colonel Richard Kemp told BBC Breakfast: “That threat of terrorist attack – whether it’s from Taliban, the Islamic State, or al Qaida – it could equally be all three of those groups.
"The fact that people are talking about Islamic State doesn’t make that the most likely threat.”
Watch: Time left for Afghanistan evacuation 'quite short' – Boris Johnson earlier on Thursday