On the other side of the jihadist ledger is ISIS-K. The group is one of many affiliates that the Islamic State established after it swept into northern Iraq from Syria in 2014, and created a religious state or caliphate the size of Britain. An American-led campaign crushed the caliphate, but more than 10,000 ISIS fighters remain in Iraq and Syria, and ISIS affiliates like the Sahel or the Sinai Peninsula are thriving.

But ISIS-K has never been a major force in Afghanistan, much less globally, analysts say. The group’s ranks have dropped to about 1,500 to 2,000 fighters, about half from its peak levels in 2016 before American airstrikes and Afghan commando raids took a toll.

Since June 2020, however, under an ambitious new leader, Shahab al-Muhajir, the affiliate “remains active and dangerous,” and is seeking to swell its ranks with disaffected Taliban fighters and other militants, the U.N. report concluded.

“They have not been a first-tier ISIS affiliate, but with the Afghan commandos gone and the American military gone, does that give them breathing room? It could,” said Seth G. Jones, an Afghanistan specialist at the Center for Strategic and International Studies in Washington.

Even as the group’s overall ranks have declined in recent years, Mr. Jones said, ISIS-K has maintained cells of clandestine fighters who have carried out terrorist attacks.

United Nations counterterrorism officials said in the June report that the Islamic State had conducted 77 attacks in Afghanistan in the first four months of this year, up from 21 in the same period in 2020. The attacks last year included a strike against Kabul University in November and a rocket barrage against the airport in Kabul a month later. ISIS-K is believed to have been responsible for a school bombing in the capital that killed 80 schoolgirls in May.

“Kabul has been the target of the majority of ISIS-K’s most sophisticated and complex attacks in the past,” said Abdul Sayed, a specialist on jihadist groups in Afghanistan and Pakistan based in Lund, Sweden.