What is ISIS-K? The Islamic State's Afghanistan affiliate is the Taliban's enemy, and killed 13 US service members in Kabul

• ISIS-K claimed responsibility for a deadly attack in Kabul that killed at least 13 US service members.
• The Taliban and ISIS-K, the Islamic State's Afghanistan affiliate, are sworn enemies.
• ISIS-K "will attempt to erode the Taliban's governance and attack the Taliban's religious legitimacy on the ground," one expert said.

President Joe Biden and his advisors in recent days warned of the potential for an attack by ISIS-K as US troops scrambled to evacuate thousands of people via the Kabul airport.

On Thursday, there was a deadly blast outside Hamid Karzai International Airport in Afghanistan's capital city. The Pentagon confirmed that at least 13 US service members were killed in the attack, and at least 18 were wounded. Dozens of Afghans were also killed and wounded.

Gen. Kenneth McKenzie, head of US Central Command, attributed the fatal explosion to ISIS-K. The terror group also officially claimed responsibility for the blast.

"The threat from ISIS is extremely real," McKenzie said during a press conference on Thursday. "We believe it is their desire to continue those attacks, and we expect those attacks to continue."

Earlier in the week, Biden said, "Every day we're on the ground is another day we know that ISIS-K is seeking to target the airport and attack both US and allied forces and innocent civilians."

ISIS-K is the Taliban's sworn enemy

ISIS-K - the Islamic State in Khorasan Province or ISIS-Khorasan - is the Islamic State's affiliate in Afghanistan. Khorasan is the historic name for the region in Central Asia that includes parts of Afghanistan.

The Taliban and ISIS-K are sworn enemies and have been fighting for years. ISIS-K views the Taliban as apostates, and not devout enough in terms of its approach to Islam. The group's leaders denounced the Taliban's takeover of Afghanistan, The New York Times reported. Along these lines, ISIS-K has an interest in conducting attacks that would induce chaos, embarrass the Taliban, and make it harder for the militant group to tighten its grip over Afghanistan.

The Taliban's victory "puts significant pressure on ISIS to demonstrate its continued relevance to global jihad, which will make ISIS more dangerous as it attempts to prove the organization's capability and relevance," Jennifer Cafarella, a national security fellow at the Institute for the Study of War, told Insider.

"Spectacular attacks in Afghanistan and attempts to conduct such attacks in the West are highly likely in coming weeks and months," Cafarella added. "ISIS will attempt to erode the Taliban's governance and attack the Taliban's religious legitimacy on the ground."

Amira Jadoon, an assistant professor at the US Military Academy at West Point, agreed, telling Insider that ISIS-K "will seek to exploit the current volatility in Afghanistan to launch attacks to increase their own political relevance and sow discord."

ISIS-K 'remains a potent threat'

ISIS-K first emerged in 2015, and staged attacks as early as April of that year. This was less than a year after ISIS took over a large swath of territory in Iraq and Syria, declaring a caliphate (which has since collapsed).

"[ISIS-K] has received support from the Islamic State's core leadership in Iraq and Syria since its founding in 2015," per a 2018 report from the Center for Strategic and International Studies (CSIS). As the Islamic State lost territory, it "increasingly turned to Afghanistan as a base for its global caliphate," CSIS said.

Militants who left the Taliban (in both Afghanistan and Pakistan) were among the founders of the group. "ISIS had sent representatives to both Pakistan and Afghanistan. They were essentially able to co-opt some disaffected Pakistani Taliban and a few Afghan Taliban [members] to join their cause," Seth Jones, an Afghanistan specialist at CSIS, told NPR.

The current leader of ISIS-K is Shahab al-Muhajir, and the group is assessed to have somewhere between 1,500 to 2,000 fighters in Afghanistan, according to a UN report from June.

The group has been "forced to decentralize," the UN said, and is primarily made up of cells spread out across the country that act autonomously but share the same ideology.

"Despite territorial, leadership, manpower and financial losses during 2020" ISIS-K "continues to pose a threat to both the country and the wider region," the UN report said.

The UN said that in the first four months of 2021 alone, ISIS-K conducted 77 attacks in Afghanistan. This marked a significant increase from the same period in 2020, where the number of attacks claimed by or attributed to ISIS-K stood at 21.

"ISIS-K has struggled to gain a large foothold in Afghanistan but it remains a potent threat and one of the most important affiliates to the ISIS global organization," Cafarella said.

Though ISIS-K experienced "significant declines in its operation capacity" since it first emerged, it "renewed its violence" over the past two years, Jadoon added.

"Its underlying sources of strength remain: it has access to a sizeable militant pool for recruitment, porous borders, and an extensive network in Pakistan," she said. "ISIS-K will continue to use the peace deal as propaganda against the Taliban, and may also try to recruit more radical members of the Taliban or other militants who feel marginalized."