CNN  — 

In the space of just a few years, an ISIS affiliate in Afghanistan has seized on the country’s instability to become one of the region’s most feared terror groups.

ISIS-K has claimed responsibility for the deadly bombing attack outside Kabul airport on Thursday, which targeted a frantic Western evacuation operation and killed 13 US troops and more than 90 Afghans. The group provided no evidence to support the claim but US officials have said it was likely behind the atrocity.

It was ISIS-K’s most globally consequential action to date and drew a promise of retribution from US President Joe Biden.

But the group, known in full as ISIS-Khorasan, has been responsible for thousands of deaths since its 2015 formation.

Its members operate in central Asia, and the group’s name comes from its terminology for the area that includes Afghanistan and Pakistan.

In 2018 it was ranked the world’s fourth deadliest terror group, claiming more than 1,000 lives, mostly in Afghanistan, according to the Institute for Economics and Peace, which monitors global terrorism annually.

Since then the group’s growth has been limited and its militants have fought the Taliban. But they have capitalized on uncertainty in Afghanistan in recent months to launch brutal attacks, and the impending withdrawal of troops by the United States threatens to give them a window in which to regain strength.

The Kabul attack claimed dozens of lives, including 13 US troops and more than 60 Afghans.

How was ISIS-K formed?

The group is a branch of ISIS, the terror group that first emerged in Syria and Iraq and, at its peak, controlled a huge stretch of territory stretching from western Syria to the outskirts of the Iraqi capital of Baghdad. The original group had its self-declared caliphate ended by US-backed forces in recent years.

But the connection between ISIS-K and its apparent parent group is not entirely clear; the affiliates share an ideology and tactics, but the depth of their relationship with regards to organization and command and control has never been entirely established.

US intelligence officials previously told CNN that the ISIS-K membership includes “a small number of veteran jihadists from Syria and other foreign terrorist fighters,” saying that the US had identified 10 to 15 of their top operatives in Afghanistan.

Its earliest members included Pakistani militants who emerged in Afghanistan’s Nangarhar province around a decade ago, many of whom had fled Pakistan and defected from other terror groups, according to the Center for Strategic and International Studies (CSIS).

Counter-terrorism analysts estimate its strength now at around 1,500-2,000, but that number may soon grow. Some captured ISIS-K fighters were being held in prisons near Kabul, which the Taliban overran as their offensive accelerated.

What do they want?

Key figures involved in the formation of ISIS-K included Taliban defectors – such as former Taliban member Abdul Rauf Aliza, who was briefly held at Guantanamo Bay and was killed in a US drone strike in 2015 after joining ISIS.

But the group has a mutual hatred of the Taliban, and attracts those with views even more radical than the Taliban.

Unlike the militant group that has seized power in Afghanistan, ISIS and its affiliates have little interest in political governance.

“ISIS believes that only God can rule. And even though the Taliban is attempting to establish an Islamic emirate, that’s not enough for ISIS,” Colin Clarke, author of “After the Caliphate: The Islamic State and the Future of the Terrorist Diaspora,” told CNN before the Kabul airport attacks took place.

In any place they control, Clarke said, ISIS-K “are going to implement extremely harsh Sharia law. And they’re going to rule with an iron fist. They want to attract and recruit the most ardent sociopaths in the country and wanton violence helps them bring other fighters into the organization that have a similar mindset.”

ISIS-K have intentions that stretch beyond the borders of Afghanistan and Pakistan; they intend to “establish a Caliphate beginning in South and Central Asia, governed by sharia law, which will expand as Muslims from across the region and world join,” according to CSIS.

And its hatred of the West, including the United States, also features prominently in their agenda. ISIS-K “has mocked and threatened the United States in its official media streams and called for lone-wolf attacks in the West,” the organization said.

What attacks has the group been responsible for?

According to UN figures, ISIS-K launched 77 attacks in the first four months of this year.

The group has carried out some of the deadliest attacks on civilians in Afghanistan, with several mass casualty suicide bombings in the capital, Kabul.

ISIS-K was believed to be behind a horrific car bombing attack outside a girls’ high school in May that killed at least 85 people.

The group was particularly active during its peak around 2018. In July of that year, an ISIS-K suicide bomber killed 128 people at an election rally in Mastung, Pakistan, one of the bloodiest attacks anywhere in the world in 2018.

According to a US State Department report, the group relied heavily on suicide bombings – the same tactic used in the Kabul airport blasts on Thursday.

The report found that in 2018, the group conducted 15 attacks in public places and killed 393 people. They included a bombing that killed 68 at a large public demonstration in Nangarhar, Afghanistan, on the anniversary of the September 11, 2001 attacks.

The group declined in strength after the US targeted its fighters across the region. But in 2019, Gen. Joseph Votel, the commander of US Central Command, told reporters during a visit to Afghanistan that they still posed a major threat both in the region and abroad.

And the Institute for Economics and Peace warned that despite the group’s decline, it was “believed to still have sleeper cells in cities such as Kabul and Jalalabad,” and its militants continued to pose a threat to the Taliban.

The group has built up a presence in eastern Afghanistan in recent years, especially in the provinces of Nangahar and Kunar. Last August, the group attacked the main prison in Jalalabad, the capital of Nangahar, in an effort to free dozens of its supporters who had been captured by the Afghan army and police.

This January, Afghanistan’s intelligence agency said it foiled an attempt by the group to assassinate a key US diplomat, Ross Wilson, in Kabul. And in June the group claimed responsibility for an attack on an international demining charity, the Halo Trust, that left 10 people dead and 16 others wounded.

CNN’s Nikki Carvajal, Jim Sciutto and Tim Lister contributed to this report.