The prime suspect for the suicide bombing at Kabul airport is the Islamic State affiliate in Afghanistan known as Islamic State Khorasan Province, Isis-K or ISKP.

Jake Sullivan, President Joe Biden’s national security adviser, said on Sunday there was an “acute” and “persistent” threat to the continuing evacuations from the Afghan capital from Isis-K – which takes its name Khorasan from that used by a series of Muslim imperial rulers for a swath of land stretching from Iran to the western Himalayas.

The warning, which focused attention on a group that has hitherto had a very low international profile, was echoed this week by British and western European officials.

Many have been worried by an intensification of attacks linked to Isis-K in recent months.

“The trajectory of ISKP has been one of resurgence after a tough time in 2019 and the first half of 2020 … but they went silent suddenly since the Taliban takeover and a possible reason for that was the group were gearing up for a new campaign,” said Charlie Winter, a senior research fellow at London University’s Centre for the Study of Radicalisation.

The crowds, planes and infrastructure at the airport provide an obvious venue for the kind of mass-casualty attack that Isis has become known for. But Winter said, the situation was also, for the group, a “perfect meeting of diverse targets” in Afghanistan: the US military, Afghans who have helped the western effort and are therefore seen as collaborators and the Taliban, which Isis-K sees as “apostates”.

Isis-K is likely to see an attack against the airport as a “great victory”.

Tore Hamming, a Danish expert in Sunni jihadism who has studied Isis, said: “They achieve several things: they hit legitimate targets (from their perspective), they send a signal of still being a force to be reckoned with and they challenge the Taliban’s state project by highlighting that the group can’t secure Kabul.”

Isis-K was founded just under six years ago after two representatives of Isis made their way to Balochistan, the south-western province of Pakistan, for a meeting with a small group of disaffected Taliban commanders and other extremists who had been fighting in the region but felt marginalised within the jihadist movement there.

The main Isis parent organisation was then approaching its zenith – seizing swaths of Syria and Iraq after a lightning campaign. The group had begun plotting its global expansion even before the victories that brought it to international attention and set about establishing affiliates all over the Islamic world.

Existing local Islamist movements often fought back against such efforts. In Afghanistan the Taliban opposed the expansion of Isis-K. So, too, did al-Qaida, the Afghan government’s forces and the US. A series of offensives resulted in heavy casualties and the death of a series of leaders.

Isis and Isis-K believe the Taliban have abandoned the Islamic faith because of their willingness to negotiate with the US, their apparent pragmatism and their failure to apply Islamic law with sufficient rigour.

Hamming said: “Like in Iraq and Syria, IS in Khorasan made the mistake of being too exclusivist … It failed to engage and cooperate with like-minded actors. In the end this approach resulted in the group facing too many enemies.”

In recent months Isis-K appears to have got a second wind, carrying out a series of lethal operations with its trademark brutality. During the first four months of 2021, the United Nations Assistance Mission in Afghanistan (UNAMA) recorded 77 attacks claimed by or blamed on Isis-K. These have targeted a wider range of targets than previously: Shia Muslims, journalists and foreigners, as well as civilian infrastructure and military personnel.

Others however say the group remains seriously degraded and that the number of attacks is not necessarily the best metric to judge its strength.

Earlier this year, specialists at the UN said that despite territorial, leadership, manpower and financial losses during 2020 in eastern provinces of Afghanistan, Isis-K was “seeking to remain relevant and to rebuild its ranks, with a focus on recruitment and training of new supporters potentially drawn from the ranks of Taliban who reject the peace process”.

In a report compiled from intelligence supplied by member nations, the UN said Isis-K territorial losses had affected the group’s ability to recruit and generate new funding. Losses have been so heavy the group was forced to offer an amnesty to members who had surrendered to the government.

“Although the group is assessed to retain a core group of approximately 1,500 to 2,200 fighters in small areas of Kunar and Nangarhar provinces, it has been forced to decentralise and consists primarily of cells and small groups across the country, acting in an autonomous manner while sharing the same ideology,” the report explained.

Since June 2020, the group has had an ambitious new leader, believed to be an Arab unlike his mainly Pakistani predecessors, and remains dangerous. One Isis-K strategy has been “positioning itself as the sole pure rejectionist group in Afghanistan, to recruit disaffected Taliban and other militants to swell its ranks”.

Many fighters come from outside Afghanistan, with a high proportion drawn from Pakistan, Tajikistan and Uzbekistan. The group retains some connections with the central Isis leadership in Iraq, though details of the relationship are unclear.

Isis-K appears to run its own media operations in direct contravention of official policy, which demands that the central leadership of Isis have control over all outlets, but may have received direct orders to conduct Thursday’s attack, as has happened with other attacks by Isis groups around the world, experts believe.

Unlike al-Qaida, which for the moment appears to have halted long-range attacks, Isis-K remains committed to striking against the “far enemy” in the west if it can, and this is likely to become more of a priority now that the immediate “near enemy” – the government in Kabul and its US protectors – have gone.

The Taliban may try to prevent such operations but will be unable to watch every corner of the sprawling, rugged country, especially as much of it is in effect ruled by local powerbrokers who make their own decisions about who does what and where.

“How much do the Taliban actually control? There is a lot of terrain in Afghanistan that Isis could take advantage of. In the immediate future they may be going for terrorist attacks to get into the news,” said Aaron Zelin, a senior fellow at the Washington institute for Near East Policy.